Human colostra IgA was hydrolyzed by trypsin for periods of 10-30 min at 50-65°C and for a 6-hr period at 37°C. More fragments were produced during shorter digestion periods at elevated temperatures than during prolonged digestion at 37°C. Molecular weights of all fragments were determined by disc electrophoresis; sedimentation constants were determined by ultracentriugation. After separation of these fragments on a Sephadex G-200 column, fractions were examined for antigenic determinants, amounts of carbohydrates, presence of J chain, and the character of subunits obtained by disulfide-bond cleavage. An F(ab)2α fragment, detected by these criteria, had a sedimentation constant of 5·1 S,a mol. wt. of approximately 115,000-132,000, and a 5·0% carbohydrate content. A small amount of Fabα was produced after hydrolysis at 60°C for 20 min. An Fcα was detected but not isolated. In fractions containing partially digested IgA and Fcα fragment, J chain was detected both immunochemically and by disc electrophoresis but was lacking in F(ab)2α fragments. Secretory component was the first polypeptide to be digested during a 20 min hydrolysis at the temperature of 60°C. © 1972.