1. A lactate dehydrogenase isoenzyme present in human spermatozoa and semen was isolated and characterized biochemically in term of its pH for optimum activity and by means of K(m) values for lactate, NAD(+) and NAD analogues. The results were compared with those obtained with the human heart-type and the liver-type lactate dehydrogenase isoenzymes. 2. The enzyme was characterized by its resistance to digestion with different proteolytic enzymes. The time for 50% digestion in terms of residual dehydrogenase activity was compared with times obtained for the H(4)- and M(4)-types.