The primary purpose of this study was to identify the mammalian tissue cultures which were most suitable for investigations of the cytopathogenicity of Trichomonas vaginalis. A recently isolated strain of the organism was inoculated into 15 different tissue cultures which were maintained in an appropriately modified growth medium. Proliferation of the protozoon was accompanied by the progressive disintegration of cell culture monolayers. Initial focal lesions consisting of detached cells and an accumulation of trichomonads gradually enlarged until the entire monolayer was disrupted. When judged by the size of the inoculum required to obtain this effect, differences among the tissue cultures were noted. An inoculum of approximately 10(3) viable trichomonads was sufficient to completely disrupt monolayers of HeLa 229, HeLa, McCoy, HEp-2, and RK-13 cells. To obtain a comparable effect with other cells, 10- to 100-fold higher levels of inoculum were required. Polyethylene glycol concentrates from culture filtrates contained a cell-detaching factor (CDF) which caused detachment and clumping of susceptible cells. Freshly seeded cells in growth medium containing CDF failed to form a monolayer. Aggregates of cells maintained for up to 1 week in the presence of CDF remained viable and formed a monolayer after being washed and suspended in normal growth medium. The activity of the CDF was not lost during 1 week of contact with the cells. The CDF may contribute to the pathogenicity mechanisms of T. vaginalis.