The antibiotic Puromycin produced from a species of Actinomyces, Streptomyces albo-niger, was tested for its trypanocidal properties against Trypanosoma equiperdum, T. equinum, T. evansi, T. rhodesiense, T. gambiense, and T. congolense infections in mice. The data presented in this report show that when treatment with Puromycin was begun approximately four hours after inoculation with these species of Trypanosoma all except T. congolense were prevented from developing. Administration of the drug four days prior to inoculation with trypanosomes did not prevent infections from progressing. When treatment was initiated at the height of infection the drug had a strong suppressive effect against all species except T. congolense. This effect was greater against those of the evansi group, T. equiperdum, T. evansi and T. equinum, than those of the brucei group, T. rhodesiense and T. gambiense. It is encouraging that this antibiotic has this powerful effect against these trypanosome infections.