Purified γ1A-globulin was prepared from a number of sera having isohemagglutinin activity against a wide range of red cell antigens. A large proportion of the saline isohemagglutinin activity of potent anti-A, anti-B and anti-A,B sera was found in the γ1A-globulin. These antibodies did not hemolyze red cells in the presence of complement. They were readily neutralized by soluble blood group specific substance. They consisted, in part, of cross-reacting antibody. Sensitivity to treatment with mercaptoethanol appeared intermediate between that of γss- and γ1M-globulin. Digestion with papain destroyed activity in saline completely, although no evidence of a molecular split was seen by immuno-electrophoresis. Digestion with pepsin reduced, but did not destroy, the isohemagglutinin activity in saline. There was no evidence of exposure of additional S groups by enzymatic digestion. No anti-D, anti-G, anti-K, anti-Fya or anti-Jka was found in purified γ1A-globulins prepared from potent antisera. Minimal anti-P and anti-Lea were demonstrated in this fraction, however.