Mammalian sperm undergo capacitation in the female reproductive tract or under defined conditions in vitro. Although capacitation is now considered to be mediated by intracellular signaling events, including protein phosphorylation, the regulation of the transduction mechanisms is poorly understood. The objective of the present study was to evaluate the importance of medium components on capacitation of porcine sperm, the appearance of an M_r 32 000 sperm protein (p32), and activity of a tyrosine kinase (TK-32). As determined by the ability of the sperm to undergo the A23187-induced acrosome reaction, pig sperm require bicarbonate and calcium but not BSA for capacitation in vitro. The appearance of p32 was assessed by immunoblotting SDS-extracted and separated sperm proteins using an anti-phosphotyrosine antibody. The appearance of p32 requires calcium, although p32 appears even in the absence of bicarbonate in the incubation medium, demonstrating that the appearance of this tyrosine phosphoprotein is not a final end point of pig sperm capacitation. An in-gel tyrosine kinase renaturation assay showed that TK-32 activity depends on calcium and bicarbonate in the incubation medium. Immunoprecipitation experiments using an anti-phosphotyrosine antibody and inhibitor demonstrated that p32 and TK-32 are different proteins. These data indicate that the signal transduction mechanisms of capacitation in pig sperm are different from those in other mammals, suggesting that certain species specificity may exist with respect to this phenomenon.