L-carnitine added to the suspension medium decreases the glucose-sustained progressive motility of human spermatozoa. Addition of 20 mM L-carnitine to the capacitation medium causes an inhibition of the occurrence of the acrosome reaction parallel to a viability enhancement and negligible changes of the cellular content of ATP. The cellular efflux of glutamate-oxaloacetate transaminase was also inhibited by L-carnitine. A possible role of L-carnitine on membrane stability and metabolism of spermatozoa is briefly discussed.