Genetic variation of human transcortin was investigated on neuraminidase-treated plasma samples by polyacrylamide gel isoelectric focusing followed by immunofixation of transcortin on cellulose-acetate strips. This procedure was applied systematically to 500 plasma samples obtained from individuals of Caucasian origin. Most samples showed one band focusing at pH 5.52. In three unrelated samples, a two-band pattern was obtained consisting of the normal band (pI 5.52) and a cathodic band of equal color intensity (pI 5.60). Family studies showed that this transcortin variant, which we call transcortin Leuven, is autosomally inherited. In a family of Polish origin two individuals homozygous for transcortin Leuven were found. A striking feature of transcortin Leuven is that it has a 2.6-fold reduced affinity for cortisol at 4 degrees C and a 3.6-fold reduced affinity at 28 degrees C. This lowered affinity is due to an enhanced dissociation rate of the transcortin-cortisol complex. As for normal asialotranscortin, a cortisol-induced charge difference of 0.15 pH unit was observed for this variant.