Total lactate dehydrogenase (LDH) activity was measured in washed sperm from 74 oligozoospermic patients and 148 normospermic men. In the same samples, an additional iso-enzyme specific for germ cells, LDH-X was also measured. Both enzymatic activities were significantly higher in sperm from oligozoospermic patients than in those from normospermic men (P less than 0.001). In oligozoospermic patients, sperm LDH and LDH-X activities were related inversely to sperm concentration (r = -0.61 and r = -0.53, respectively). In normospermic men, this inverse relationship was less evident (r = -0.28 and r = -0.25, respectively). After separation of sperm on a discontinuous gradient of Percoll, LDH and LDH-X activities were increased in the non-migrated-sperm fractions and reduced in the migrated-sperm fractions in both groups of subjects when compared to the initial semen specimens. In addition, migrated sperm from oligozoospermic and normospermic men did not differ in their LDH and LDH-X activities. Although the possible contribution of immature germ-cells to these findings remains to be defined, the data suggest clearly that high levels of intracellular LDH and LDH-X are a biochemical feature of sperm from oligozoospermic subjects.