The present study was undertaken to determine whether live T. vaginalis degrades human secretory IgA, serum IgA and IgG molecules. Human immunoglobulins were exposed to live trophozoites, parasite lysate, and excretory-secretory product (ESP) of T. vaginalis. To determine the fragmentation of immunoglobulins, the reaction sample was subjected to SDS-PAGE and EITB, and peroxidase conjugated antihuman IgA and IgG were used as probes. Live trophozoites degraded secretory IgA. Serum IgA and IgG, and degradation were pressed forward by the prolongation of the incubation time and by increasing the number of trichomonads respectively. Also the lysates and ESP of trichomonads degraded IgA and IgG. The cysteine and serine proteinase inhibitors such as E-64, antipain, iodoacetic acid, iodoacetamide, TLCK reduced the ability of cleaving immunoglobulins. The proteinase activity and cytotoxicity of T. vaginalis to HeLa cells were decreased when live T. vaginalis was treated with metallo-proteinase inhibitor as well as cysteine and serine proteinase inhibitors. These results suggest that proteinase secreted from live T. vaginalis may play a part role in host pathogenesis by T. vaginalis, and the cleaving ability of host immunoglobulins by the proteinase may contribute as a one of immune evasion mechanism for parasite survival in the host.