Enzyme activity against hippuryl-L-arginine was studied in cultured skin fibroblasts from controls and cystic fibrosis patients. The enzyme had a lysosomal distribution and an acid optimum of pH 4.5 with little or no activity present above pH 7.0. Dithiothreitol was required for full activity and the kinetics of thiol activation were different for the control and cystic fibrosis enzyme. The properties and lysosomal distribution of the enzyme indicated that it was a carboxypeptidase B. Substrate affinity, thermolability, pH stability, the fall and rise in activity with subculture, the cyclical pattern of activity through serial passage and the level of activity were similar for the control and cystic fibrosis enzyme.