Proteolytic activity, defined as the hydrolysis of peptide bonds involving the carboxyl group of L-arginine, in plasma of patients with cystic fibrosis, heterozygotes, and control subjects has been assayed using a fluorometric method with protamine as the substrate and fluorescamine as the reagent. The mean total proteolytic activity in plasma of patients with cystic fibrosis was approximately one-half the mean total activity in control subjects and heterozygotes. The mean proteolytic activity inhibited by soybean trypsin inhibitor in plasma of patients with cystic fibrosis was approximately one-third that of control subjects and heterozygotes. The relationship of arginine esterase activity to proteolytic activity was investigated. The pH optimum and action of reversible and irreversible inhibitors were similar for both activites, suggesting that the arginine esterase activity and proteolytic activity represent similar catalytic entities. These findings are consistent with our hypothesis that the basic defect in cystic fibrosis may reside in the deficieny of a proteolytic enzyme which results in the accumulation of the various cationic macromolecular "factors" described by other investigators in serum of patients with cystic fibrosis. The demonstration of a deficiency of proteolytic activity as assayed by the hydrolysis of protamine, a cationic polypeptide, could explain the presence of ciliotoxic cationic protein or polypeptide factors in serum of patients with cystic fibrosis and may, in some unknown manner, be related to the clinical manifestations of the disease.