No substantial creatine kinase (CK) (E.C.2.7.3.2) activity was found in the lysates of highly purified human sperm. In these lysates, even a very sensitive RIA method was unable to reveal significant amounts of the isoenzyme CK-BB. Whole human semen and sperm lysates contained ATP, ADP and AMP but no specific substrates (creatine phosphate or creatine) for CK. A direct, specific effect on anti-CK-BB antibodies on human sperm mobility, agglutination, viability was not found.