In this placebo-controlled study the effect of short term, low-dose prednisolone treatment on sperm-bound antibody levels was investigated using an objective flow cytometric (FCM) measurement. Twenty infertile men, positive for antisperm antibodies by the mixed antiglobulin reaction (MAR) test, were treated with either 20 mg/day prednisolone (n = 10) or placebo (n = 10) for 3 weeks. The two groups were similar with respect to sperm count, motility and antibody levels. The proportion of spermatozoa positive for IgG and IgA antibodies and the antibody load, expressed as the number of IgG and IgA antibody molecules bound per spermatozoa were measured by FCM. The study showed that, overall, the antibody levels measured before and after treatment with low-dose prednisolone or with placebo were not statistically different. However, in the prednisolone group a marked decrease in the proportion of spermatozoa positive for IgG and IgA antibodies was observed in two of the patients. An examination of the FCM data of these two patients revealed that, in their ejaculates, only the spermatozoa with low to moderate amounts of sperm-bound antibodies had responded to prednisolone treatment, while spermatozoa with high levels of sperm antibodies had remained resistant to prednisolone. These results point to the existence of an as-yet-undefined subset of prednisolone-responsive, antibody--positive spermatozoa.