Demethylchlortetracycline (DMCT) is an antibiotic which chelates with calcium ions and induces a characteristic golden-yellow fluorescence in bones and teeth. A single oral dose of 10 mg/kg of body weight was administered to 11 captive coyotes (Canis latrans). Portions of bone and tooth dentin from these animals fluoresced under ultraviolet light. Gross fluorescence was most apparent on the mandible, with lesser amounts on the pelvis, femur, and tibia. Intensity and quantity of fluorescence was greatest in younger animals. The degree of fluorescence was similar in animals killed at 2 months and 5 months posttreatment, indicating that the compound is effective as a long-term label. The field testing of antifertility agents to reduce coyote populations (Balser 1964) presents certain difficulties, one of which is identification of individual animals which have consumed treated baits. Incorporation of a chemical marker into antifertility baits would alleviate this problem, provided the label is effective in a single oral dose and persists in the animal for the duration of field tests (5 months). The tetracyclines are a group of antibiotics which chelate with the calcium ions in bones and teeth. Their presence in these tissues is indicated by the emission of a characteristic golden-yellow fluorescence under ultraviolet light. This phenomenon was reported by Milch et al. (1957, 1958) who showed that tetracycline-induced fluorescence in soft tissues disappeared within a matter of hours, but persisted in long and flat bones for long periods of time. They noted that deposition of tetracycline was limited to areas of new bone growth. Thus the intensity and amount of fluorescence bound to the bone was directly related to the age of the animal (Milch et al. 1958: 906). Frost et al. (1961), Storey (1963), and Ibsen and Urist (1964) have reported and summarized similar findings. An excellent review of the tetracycline literature has been compiled by Johnson (1964). Aside from their value as antibiotics, the tetracyclines have also been used to diagnose neoplastic tissue, to study normal bone p ysiology, and to label the teeth of fur seals (Yagi et al. 1963) and the bones and scales of fish (Weber and Ridgway, 1962). The tetracyclines are poorly retained by the skeleton when given by oral route, but increased uptake can be induced by injection. Kelly and Buyske (1960:148) and Buyske et al. (1960:153) estimate that less than 0.1 percent of tetracycline was retained in the bones of rats 1 week after oral administration as compared to 3-6 percent retention by intraperitoneal route. Data presented in this paper show that even small oral doses are sufficient to cause grossly visible bone fluorescence. The authors are indebted to Dr. John Bowne and his staff, USDA, Animal Disease Research Laboratory, Denver Federal Center, for the use of microphotography equipment, and to Herbert Brusman, Keith LaVoie, and Lewis Sparks, Denver Wildlife Research Center, for help during cerain phases of the study. Thanks are due Charles Loveless for statistical analysis of data.