Methyl 7-diethylamino-4-hydroxy-6-n-propylquinoline-3-carboxylate, amquinolate, initially showed great promise as a broad-spectrum agent in extensive evaluation studies against 120 laboratory and field strains of coccidia. Subsequently, further tests were conducted with 12 field strains obtained from commercial poultry farms where a related quinoline coccidiostat reportedly had failed to control coccidiosis. The 12 strains were resistant to four new quinoline coccidiostats: amquinolate, buquinolate, MB 1968). As part of an extensive study of this compound we tested it against 120 strains of coccidia. Of these, 24 strains (representing 8 species of Eimeria) had been propogated from single oocysts that had previously been isolated from field cultures. The remainder were field strains from widely separated geographical areas; they included various species of Eimeria and had been implicated in clinical coccidiosis. Amquinolate, at a concentration of 0.003% in the diet, was fully effective against 111 (92%) of the strains; and a combination of 0.003% amquinolate and 0.01% amprolium was effective against 120 (100%) of them. Upon completion of this series of tests, 12 field cultures were received from commercial poultry facilities in which another quinoline coccidiostat had allegedly failed to give satisfactory control of coccidiosis. It was considered of special interest to test several quinoline coccidiostats against these strains to determine if drug resistance was involved, and if so, to determine whether the strains were resistant to more than one quinoline compound. This paper reports the results of these tests, together with the results of tests designed to induce resistance to amquinolate by means of exposure to the drug under laboratory conditions. MATERIALS AND METHODS For measurement of effects of drugs, 2-week-old sexand weight-balanced whitecross chicks in groups of 10 were fed a standard Received for publication 9 August 1968. laboratory ration to which drugs were added just prior to use. The uninfected and infected control birds were fed the basal ration. On the second day of the test the chicks were inoculated orally with sporulated oocysts. The number of oocysts given per bird depended upon the species being used in a given test as follows: E. tenella, 50,000; E. necatrix, 50,000; E. brunetti, 100,000; E. maxima, 100,000; E. acervulina type, 200,000; mixed cultures, 200,000 to 250,000. Several criteria were employed for evaluation of efficacy of a drug. These included observations and records on the mortality rate, growth, severity of pathological lesions, and number of oocysts produced. Coccidial lesions were scored according to the following system: 0 = normal, 1 = detectable, 2 = moderate, 3 = marked, 4 = maximal. To obtain estimates of oocyst production, intestinal tissue or fecal material was homogenized in a blender. Appropriate aliquots were diluted, placed in a hemacytometer, and counted. For each group of birds all of these criteria were combined to yield a single criterion of efficacythe anticoccidial This number indicates the overall relative activity of the agent tested in each group of birds. The index is the sum of (1) the per cent survival of the birds plus (2) the per cent relative weight gain minus (3) the lesion index, and minus (4) the oocyst index. Each component of the index for a group of chicks is computed as follows: (1) The per cent survival (0-100%) contributes the corresponding number of points (0-100). (2) The per cent relative weight gain, calculated by comparing the average weight gain of the group to the average weight gain (as 100%) of the noninfected nonmedicated group, contributes the corresponding number of points (0-100, or more or less). (3) The average coccidiosis lesion score (04.0) is multiplied by 10 to become the corresponding assigned lesion index (0-40). (4) The number of millions of oocysts per bird ( 40) is (a) in the case of infections with intestinal species, compared to the oocyst count (as 100%) for the infected nonmedicated control group to obtain an observed per cent of