We have developed a sensitive assay for the measurement of alanine:glyoxylate aminotransferase (EC 2.6.1.44) activity in human liver. The assay is partly automated, and takes into consideration the sensitivity of the reaction to pH and to glyoxylate concentration. It is less subject to interference from other enzymes utilizing glyoxylate and to chemical interference from glyoxylate itself and can therefore be used without correction for cross-over by glutamate:glyoxylate aminotranferase (EC 2.6.1.4). The assay allows clear discrimination between normal and affected livers and is sufficiently sensitive to measure enzyme activity in fetal liver samples. Enzyme activity ranged from 17.9 to 38.5 mumol/h/mg protein in control livers (n = 9) and 0.8 to 9.5 mumol/h/mg protein in 30 of 39 hyperoxaluric patients studied. Normal alanine:glyoxylate aminotransferase activity (from 22.8 to 45.5 mumol/h/mg protein) allowed exclusion of primary hyperoxaluria type 1 in the other nine hyperoxaluric patients.