AbstractA major inhibitor of acrosin in rhesus monkey and rabbit oviduct fluid, isolated by isoelectrofocusing in sucrose gradients, displayed a broad peak in the acidic region of the column and was demonstrated to contain secretory IgA specific for acrosin. Its identity was established by immunodiffusion, by the removal of acrosin inhibition with antisera to IgA (α‐chain), and by its correct molecular weight during ultracentrifugation. Purified human serum IgA also inhibited rabbit, rhesus monkey, and human acrosins, but neither purified human IgG nor IgM had any inhibitory effect on these acrosins. Neither oviduct fluid secretory IgA nor purified human serum IgA inhibited the activity of bovine pancreatic trypsin. The high specificity of secretory IgA for acrosin and its presence in every rabbit and rhesus monkey oviduct fluid specimen examined suggests a possible regulatory role for this antibody in reproducti