Fifty-nine patients referred for testing of galactose-1-phosphate uridyl transferase (GALT) activity were evaluated by a combination of radioisotopic assay and an improved high-resolution isoelectric focusing (IEF) method for determination of individual genotypes. The majority of evaluations (43 of 59) were undertaken because of an abnormal fluorometric GALT screening assay conducted as part of a state newborn galactosemia screening program or as a diagnostic test for an infant with one or more clinical signs of galactosemia. Only 12 of the 43 patients with an abnormal fluorometric screening test were found to have less than 3% GALT activity by quantitative assay, no GALT-staining by IEF, and a clinical history consistent with classical galactosemia. The majority of the remainder (24 of 43) were found to be heterozygotes for galactosemia, the Duarte variant, or both. IEF was found to be a fast and accurate technique for assignment of GALT genotypes in conjunction with quantitative enzymatic assay.