Borrelia burgdorferi B31 cultured at 28 degrees C was used to assay the effects of tick hemolymph on outer surface protein C (OspC) surface expression. Using immunofluorescence, OspC surface expression was shown to increase in cultures treated with hemolymph from both Ixodes scapularis and Dermacentor variabilis, while control samples (non-hemolymph treated) showed no increase in the levels of OspC. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis also showed an increase in OspC from spirochetes exposed to tick hemolymph. In addition, a lower-molecular-mass protein band of unknown identity was also enhanced in the hemolymph-exposed spirochetes. The significance of these effects of tick hemolymph on B. burgdorferi outer surface protein expression is unknown.