LDH isozymes were studied in human testes from prepuberal and mature aged individuals in monolayer testicular cultures, long-term cultivated seminal tubules, spermatozoa, seminal plasma, serum and erythrocyte hemolysate. The LDH-X fraction was found in spermatozoa, testes and cultivated seminal tubules at maturity in contrast to testes and monolayer testicular cultures from prepuberal individuals. LDH 5 was absent in spermatozoa and erythrocyte hemolysate. The authors are of the opinion that LDH-X fraction can serve as a biological enzymatic marker in human spermatozoa.