The oxidative stress response induced in Trichomonas vaginalis by exposure to various concentrations of hydrogen peroxide (H2O2) was traced by metabolic labeling and monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography of trichloroacetic acid-precipitated proteins. Concentrations of H2O2 > 450 µM decreased incorporation of radiolabel into protein and altered protein synthesis resulted in a banding pattern similar to the heat shock protein profile. The proteins produced by oxidative stress included molecules with average molecular masses of 145–165, 92–84, 66, 43–46, and 35–36 kD. Oxidative stress induced changes in T. vaginalis protein synthesis slowly. Full conversion to oxidative stress response occurred within 150–180 min after stress initiation. The oxidative stress responses of three strains freshly initiated from stabilates were compared with the responses of the same strains cultured in vitro for extended periods (culture adapted). Both freshly initiated and culture adapted T. vaginalis of the three strains synthesized the 92–84- and 66-kD heat shock proteins but differed in the synthesis of the 74–75-, 43-, and 35–36-kD molecules. Culture adaptation did not modify the oxidative stress response of the three strains tested.