Twenty calves with natural gastrointestinal parasitisms were used in an evaluation of the anthelmintic activities of cambendazole. On the 1st experimental day, half of the calves were given the drug in boluses at about 25 mg/kg. The remaining calves were used as nonmedicated controls. Five treated calves and 5 control calves were killed 3 days later. The other 5 treated calves were given a second medication at the same rate 14 days after the initial treatments and killed 3 days later along with 5 control calves. The geometric mean of the nematode population in each calf in the first control group was 22,350, and 19,700 for each calf in the second control group. Adult Haemonchus placei, Ostertagia ostertagi, Trichostrongylus axei, and Cooperia punctata comprised most of the populations although a few Bunostomum phlebotomum, Capillaria bovis,Trichuris ovis, and Oesophagostomum radiatum were present along with a few immature nematodes. The overall reductions of immature and adult nematodes in the calves in both treatment groups exceeded 99%. These reductions were highly significant (P < 0.01). No statistical differences were found between the 2 control groups or between the 2 treatment procedures. Cambendazole [5-isopropoxy-carbonylamino2-(4-thiazolyl) benzimidazole, Merck & Co., Rahway, N. J.] has been described (Hoff et al., 1970) and reported to be an effective anthelmintic for sheep and calves (Egerton and Campbell, 1971; Egerton, Eary et al., 1970; Benz, 1970). This project was conducted to obtain additional information about the efficacy of the drug when calves were treated only one time or treated twice at a 2-week interval. MATERIALS AND METHODS The experimental calves were of mixed breeding and sexes. They were initially selected in southern Florida on the basis of their exhibiting clinical signs of parasitisms and the finding of numerous nematode ova during fecal examinations. The calves were transported by truck to this laboratory and confined to a paddock of approximately 2 acres for 8 days after their arrival. When the trial was started they were kept in a pen with a concrete floor which was cleaned daily. A limited supply of dry forage was present in the paddock. Coastal Bermuda hay and water were provided ad lib. throughout the experiment. Fecal samples were obtained the day after the calves were delivered. Eggs per gram of feces Received for publication 18 August 1970. * Approved by the Publications Committee, School of Veterinary Medicine, Auburn University, as Publication No. 1080. t Supported in part by the Alabama Agricultural Experiment Station, Auburn, Alabama, and by Merck, Sharp & Dohme Research Laboratories, Rahway, New Jersey 07065. (EPG) were estimated using the modified McMaster technique (Whitlock, 1948). Fecal cultures were prepared and larvae were identified 1 week later (Keith, 1953). Twenty of the calves were used in the trial. Criteria employed for their final selections i cluded high EPGs and the presence of infective (3rd-stage) larvae of Haemonchus, Ostertagia, Trichostrongylus, and Cooperia in the fecal cultures. The calves were placed into Groups I through IV on a random basis. They weighed an average of 158, 129, 151, and 151 kg respectively at the start of the trial. Ten calves (Groups II and IV) were given cambendazole in boluses at the onset of the trial and half of these calves (Group IV) were given another treatment 2 weeks later. Only half a bolus was given individually since a complete bolus contained 6.81 g of the drug. The average rates of administration were 26.4 mg/kg (range 22.0 to 35.8 mg/kg) for the calves in Group II and 22.5 mg/kg (range 19.5 to 27.2 mg/kg) for those in Group IV. The other 10 calves (Groups I and III) were used as nonmedicated control animals. Three days after the initial treatments, the calves in Groups I and II were killed. The other calves (Groups III and IV) were killed 3 days following the second medication. Fecal examinations were conducted at the times of treatments and on the days necropsies were performed. The postmortem procedures included the excision of the gastrointestinal tract and the prompt ligation of the fundic, pyloric, and ileocecal junctures. The abomasum, small intestine, and large intestine (including the cecum) were removed from the peritoneal cavity, placed in separate pans and labeled. These organs were opened lengthwise and the mucosal surfaces washed. The contents and washings from each organ were combined and placed in separate containers. Formaldehyde was added