Studies on a naturally occurring human antibody active against mouse landschutz ascites tumor cells: II. Identification of the antibody as igm and its partial purification

1. The cytotoxic antibody in normal human serum, active against mouse tumor cells, was identified as a 19S macroglobulin by means of Sephadex gel filtration, DEAE‐cellulose chromatography and density‐gradient centrifugation procedures. 2. A purified IgM fraction was isolated from normal human serum. The cytotoxic activity of this fraction, as tested against mouse tumor cells, was enriched 20–80 times over that of the unfractionated serum. The purified IgM showed mainly one precipitin line on immunoelectrophoretic analysis. In the ultracentrifuge the 19S peak comprised about 85% of the total protein.