Bovine seminal plasma (BSP) contains four similar acidic proteins, previously designated as BSP-A1, BSP-A2, BSP-A3, and BSP-30-kDa. These proteins are secreted by the seminal vesicles and coat the surface of the spermatozoa after ejaculation. The binding site of BSP proteins on the sperm surface has been identified as choline phospholipids on the plasma membrane. This study was undertaken to determine whether BSP proteins modulate capacitation of bovine spermatozoa induced by heparin. Bovine epididymal spermatozoa were washed and incubated in buffer containing BSP proteins and then washed and incubated with heparin. The percentage of capacitated spermatozoa was determined under the microscope after the acrosome reaction has been initiated with the addition of lysophosphatidylcholine. The results demonstrated that epididymal sperm undergo the acrosome reaction only in the presence of BSP proteins. This effect was concentration-dependent and reached a maximum level of a 3-5-fold increase at 20-40 micrograms/ml BSP protein concentrations. In contrast, ribonuclease (purified from bovine seminal fluid) or seminal fluid proteins depleted of BSP proteins (by sequential absorption of BSP proteins on gelatin-Agarose and DEAE-Sephadex columns) showed no significant potentiating activity. The purified BSP proteins were more active than crude alcohol precipitates of bovine seminal plasma. These results indicate that BSP proteins are regulatory factors of capacitation.