A protein with an isoelectric point (pI) of 5.48 was found to be deficient in plasma from most cystic fibrosis (CF) homozygotes and obligate heterozygote carriers of CF as compared with normal control plasma. Purification of the protein with a pI of 5.48 from normal plasma was performed using ammonium sulfate precipitation, DEAE-cellulose and CM-cellulose chromatography, Sephadex G-200- gel filtration, starch block electrophoresis, and Sepharose 4B gel filtration. The purified protein migrated as a single band on polyacrylamide gel electrophoresis, and displayed a single arc on immunoelectrophoresis against polyvalent antiserum to whole human serum. Results from various techniques used in its characterization indicate that this protein is a fragment of alpha2-macroglobulin (alpha2M) which is derived from alpha2M by proteolytic cleavage of intact alpha2M subunits. Quantitation of alpha2M levels in plasma indicated no significant differences between levels of alpha2M in CF homozygote, obligate heterozygote carrier, or normal control plasma samples. Quantitation of arginine esterase activity in plasma treated with cloroform and ellagic acid indicated that both the total arginine esterase activity and that fraction of arginine esterase activity inhibited by soybean trypsin inhibitor (SBTI) were decreased in most CF homozygote and obligate heterozygote plasma samples relative to normal control values. The results of this study indicate that plasma samples from CF homozygotes and obligate heterozygote carriers for CF show deficient proteolytic cleavage of alpha2M as compared with normal control plasma, and suggest that a structural abnormality in alpha2M or a deficiency in plasma proteolytic activity may be responsible for this deficiency in proteolysis.