(S)-alpha-Chlorohydrin interferes with glycolysis in bovine spermatozoa whereas the (R)-isomer is ineffective. The action of the (S)-isomer, which involves inhibition of the reaction catalysed by glyceraldehyde 3-phosphate dehydrogenase, is not immediate but is evident only after a brief period of incubation with the spermatozoa. This inhibitory action is prevented when glycerol is present suggesting that the mechanism of action of (S)-alpha-chlorohydrin requires its oxidation to (S)-3-chlorolactaldehyde which is the active metabolite. Addition of racemic 3-chlorolactaldehyde to bovine spermatozoa caused immediate inhibition of glycolysis. It is proposed that the action of (S)-alpha-chlorohydrin in bovine spermatozoa is similar to that observed in the spermatozoa of other species in being a two-stage process; first, its oxidation to (S)-3-chlorolactaldehyde, and then inhibition of the glycolytic enzyme by this metabolite.