The activity of the enzyme lactic dehydrogenase (LDH) in semen, seminal plasma and spermatozoal fractions of buffalo bulls and the enzyme kinetics of LDH in the seminal plasma of bulls and buffalo bulls were studied. Highest activity of the enzyme was found in the midpiece fraction followed by the tail fraction, and a very low activity was found in the head fraction. Highly significant differences were observed between bulls ( P < 0.01) in the distribution of the enzyme in the tail fraction. The activity of the isozyme LDH 1 was greater than that of LDH 5. The enzyme kinetics in the seminal plasma followed a first order reaction and the activity was linear up to 60 min. The pH optimum was 7.1 and the temperature optimum was 37°C. Vmax and Km values of LDH were found to be higher in bull seminal plasma than in buffalo seminal plasma whereas the reverse was true for Ea and Q 10 values in these species.