A recently described cytotoxic monoclonal antibody (mAb 1D5) raised against Blastocystis hominis isolate B, was tested for reactivity with 13 different isolates of Blastocystis. The isolates used were previously isolated from humans, rats and reptiles and were maintained as axenised cultures throughout the course of this study. Five B. hominis isolates (B, C, E, G and H) were found to react with mAb 1D5 in immunoblotting studies and the indirect fluorescence antibody test. The pattern of fluorescence observed for all five isolates was diffuse and patchy. Immunoblotting studies revealed that mAb 1D5 reacted with a 29-30-kDa protein found in all five isolates. Results of a cytotoxic assay showed that the mAb exhibited a complement-independent cytotoxic effect on all the exposed isolates. Microscopic observations showed differences in morphology between the Blastocystis cells exposed and unexposed to mAb. Acridine orange staining performed on both exposed and unexposed cells showed similar internal structures when viewed under fluorescence microscopy.