Determinations of aspartate and alanine aminotransferase activity in human semen were carried out by the spectrophotometric and colorimetric methods. It was found that the occurrence of interfering substances in semen makes the spectrophotometric method unsuitable without prior partial purification of the enzyme. The mean activity determined colorimetrically in 417 samples of human semen was 320 Sigma-Frankel units/ml (s.e.m. ±8, range 10 to 1070) for aspartate aminotransferase and 30 units/ml (s.e.m. ±3, range 0 to 97, n = 40) for alanine aminotransferase. As regards aspartate aminotransferase, the activity of this enzyme was highest at +60° C. Pyridoxal phosphate increased the activity, particularly if added to semen samples with initially low activity. Gel electrophoresis of seminal plasma showed one band migrating towards the anode. However, in homogenized spermatozoa the bulk of enzyme migrated towards the cathode and only a weak band could be demonstrated on the side of the anode.