Because there are conflicting data regarding the effect of different temperatures and durations of storage on the stability of the activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT), a new study has been conducted to re-examine this important issue. Blood obtained from patients with varying aminotransferase levels was centrifuged, the resultant serum was divided into aliquots, the samples were stored at room, refrigerator, and freezer temperatures, and the aminotransferases measured on days 0, 1, 2, 3, 4, and 30. In all but one circumstance, both AST and ALT activities declined markedly beginning within 24 hr of venipuncture; the temperature of storage did not significantly affect the rate or degree of loss of enzyme activity. The exception was the evidence that ALT activity in samples obtained from individuals with initially normal values showed a rise during the first 3 to 4 days, followed thereafter by a decline to below baseline values. Thus, to ensure accuracy of aminotransferase measurement, testing of samples should be conducted on the day of venipuncture.