As a part of continuing investigations into the factors of trichomonad pathogenicity, a histopathologic study has been conducted of subcutaneous lesions produced in C57B1/6J mice by various strains of Trichomonas vaginalis (TVC1, TVC, TV 1, TV 3) and of T. gallinae (JB, TG, YG). The basic mechanisms of reaction are defined and their relationships discussed. Relative pathogenicity of the various strains is assessed on the basis of host tissue reaction. The progressive stage of the reaction to the medium utilized (control) involves the first 8 hr; thereafter, resolution ensues. The process is that of acute inflammation terminating as a well-delineated foreign body reaction. With the introduction of trichomonads, the added duration of the stage of progression continues for periods of time which vary with the strain of organism. In the case of the most virulent strains, this progression is actively continuing at the time of termination of these experiments (2 weeks). The mechanisms of progression in all strains studied are orderly and involve: influx of polymorphonuclear leukocytes; multiplication of the parasites; death of the leukocytes; destruction of host tissues with lysis of the abscess wall, edema of the surrounding tissues, and spreading of the flagellates; influx of leukocytes; remultiplication of the organisms and continuation of the cycle. This cyclic progression results in pure mantles of either leukocytes or of trichomonads formed against the inner wall of the injection pocket. Both the precise picture and time sequence vary with the strain of the organism and are related to its pathogenicity. Strains studied which were handled less efficiently by the mouse were also more pathogenic to their natural hosts. Recently one of the authors (Honigberg, 1961) published a report on the comparative pathogenicity of strains of the human genital flagellate, Trichomonas vaginalis Donne, and of strains of the morphologically closely related avian parasite, Trichomonas gallinae (Rivolta). The investigation involved the study of gross pathology of lesions produced in purebred mice Received for publication 26 July 1962. *This investigation was supported by research grants CT-805(Cll), from the National Cancer Institute, and E 742, from the National Institute of Allergy and Infectious Diseases, Public Health Service. t The authors are indebted to Drs. James E. Cavanagh, Leon Jacobs, A. B. Kupferberg, and Robert Stabler for providing the several strains of parasites employed in this investigation. The technical assistance of Mesdames M. C. Livingston and M. T. McLure, and of Mr. Jagabandhu Mitter is also acknowledged. by subcutaneous inoculations of several axenically m intained strains of each of the two trichomon d species. The relative pathogenicity of the strains to mice was evaluated by statistical comparison of the mean volumes of such lesions. There appeared to be a close correlation between the volumes of the abcesses caused by the parasite strains in the experimental hosts and the degree of pathogenicity observed in the natural hosts infected either naturally or experimentally with these