Unlike normal human cells, cultured fibroblasts from patients with argininosuccinic aciduria cannot synthesize arginine from citrulline because they have a deficiency of argininosuccinic acid lyase (ASL). We have found that V79, a Chinese hamster cell line, cannot grow on citrulline. Although these cells show a normal uptake of citrulline and have levels of ASL comparable to a human cell line (HeLa) which can grow in citrulline-containing medium, V79 cells have less than 5% of the argininosuccinic acid synthetase (ASS) activity of HeLa and cannot convert citrulline to argininosuccinate and thence to arginine. When heterokaryocytes are formed between V79 and a human cell line derived from a patient with ASL deficiency, complementation takes place and citrulline is incorporated into cell protein, presumably after having been converted to arginine. This is the first time that a genetic defect of the urea cycle has been corrected in human cells.