Good sperm motility in normozoospermic men was associated with high bound and total sialic acid (SA) and pyruvate concentrations in the seminal plasma, as well as high endogenous (sperm) pyruvate concentrations. The presence of sialidase in the seminal plasma was also demonstrated for the first time. The results suggest that SA is metabolized to pyruvate by a N-acetylneuraminic acid (NANA)-aldolase in the seminal plasma and that pyruvate subsequently serves as an energy source for sperm.