A procedure is described for the purification of procarboxypeptidase A from acetone powder prepared from extracts of pancreas glands of the spiny Pacific dogfish. The apparently homogeneous zymogen is a monomer in contrast to the procarboxypeptidase A complex isolated from the bovine pancreas. Physical and chemical properties of dogfish procarboxypeptidase A were found to be similar to those of succinyl fraction I, the chemically modified, monomeric precursor of bovine carboxypeptidase A. Significant differences, however, exist in the number of sulfur-containing residues in each zymogen. Dogfish procarboxypeptidase A was rapidly activated by bovine trypsin or Nagarse in the presence of Ca2+ ions (up to 0.5 m). Activation by chymotrypsin resulted in the generation of only about 18% of the total potential activity of the zymogen. Calcium ions increased neither the efficiency nor the extent of activation by chymotrypsin. Dogfish carboxypeptidase A was isolated following the activation of purified procarboxypeptidase A with bovine trypsin. The enzyme appeared to be homogeneous in the ultracentrifuge and resembled its bovine counterpart in physical, chemical, and catalytic properties. Dogfish procarboxypeptidase A was also found to have catalytic activity toward ester and peptide substrates. Comparison of the catalytic properties of the enzyme and the zymogen reveals that the product of the activation process probably has a more efficient binding site rather than a more effective catalytic site.