Lysates of lymphocytes, isolated from whole blood, and Epstein-Barr virus transformed cultured lymphoblasts catalysed the transamination of 4-aminobutyric acid with 2-oxoglutaric acid as co-substrate. 4-Aminobutyric acid aminotransferase activity in lymphocyte and lymphoblast sonicates derived from 12 unrelated control individuals (6 each) was 39 +/- 19 pmol min(-1) (mg protein (-1] (mean +/- 1 SD). Activities in lysates of both types of cell derived from a Flemish patient were less than 3% of control. 4-Aminobutyric acid aminotransferase activity in sonicates derived from the parents and a healthy sibling were 15-37% of the control mean for lymphocytes and 13-20% of the control mean in lymphoblasts, respectively. Km values in a control lymphoblast sonicate were 0.63 and 0.08 mmol L(-1) for 4-aminobutyric and 2-oxoglutaric acids, respectively. These data indicate that the parents and healthy sibling are heterozygous and the patient is homozygous for a defective gene responsible for 4-aminobutyric acid aminotransferase deficiency, and that inheritance is autosomal recessive.