1. The activity of the leucine catabolic pathway was measured by quantitation of the radiolabeled glutamic acid produced by intact cultured fibroblasts incubated with radiolabeled leucine. 2. Five normal cell lines produced an average of 179 +/- 47 pmol glutamic acid/mg cell protein/hr. 3. Six cell lines with defects in one of the enzymes of the pathway showed activities of 0-40 pmol glutamic acid/mg cell protein/hr. 4. Each of the cell lines with an enzyme defect produced a characteristic pattern of radiolabeled organic acids. In every case, the amounts of these organic acids produced was less than the amount of glutamic acid produced by normal cells. None of the cell lines accumulated CoA esters.