The existence of an unusual isozyme of lactic dehydrogenase (“band X” isozyme) in postpubertal testes and in normal spermatozoa has recently been described. The possibility that some cases of human male infertility with pathological or disturbed spermatogenesis might be due to a deficiency of this isozyme constitutes the subject of present investigations. The “band X” was observed in spermatozoa in all cases of normospermia, as well as necrospermia and moderate to severe oligospermia. These results suggest that this isozyme cannot be considered an index of viability or fertilizing ability of spermatozoa. Any possible specific metabolic function which it fulfills is at present unknown. The “band X” could not be demonstrated in ejaculates of cases with aspermia or azoospermia, or in testicular biopsy samples which showed, on microscopic examination, an absence of mature spermatozoa. The possibility that the “band X” is formed late in the process of spermatogenesis, possibly during the development of mature spermatozoa, is discussed.