Isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with acrylamide copolymerized with gelatin (substrate-SDS-PAGE) were combined to evaluate the proteinases of both long-term-grown and fresh isolates of Trichomonas vaginalis. This two-dimensional substrate-SDS-PAGE resolved as many as 23 distinct proteinase activities in several isolates, and proteinases had relative molecular masses between 23 and 110 kilodaltons (kDa). Isoelectric points (pI) of proteinases ranged from 5.7 to 7.0. Overall, the various representative proteinase profiles were similar among those of long-term-grown and fresh isolates, although heterogeneity existed among several cysteine proteinase activities. Pattern changes were detected in fresh isolates passaged over several weeks, showing the ability of proteinases to be differentially expressed and to undergo phase variation. The two-dimensional proteinase patterns were very reproducible for isolates analyzed over a certain period of time before expression of some proteinases varied. The heterogeneity and differential expression of certain proteinases were not coordinated with phenotypic variation of already characterized immunogens and adhesins. Data suggesting that a 43-kDa proteinase resided on the parasite surface were obtained on the basis of removal of activity following pronase or proteinase K treatment of live organisms. Finally, immunized experimental animals produced antibody to many T. vaginalis proteinases, which indicates the immunogenic nature of trichomonad proteinases.