An IgMκ cold agglutinin reacted with cord blood as well as with adult red blood cells (RBC). The agglutinin reacted strongly with papainized and neuraminidase (RDE)-treated RBC. It resembled anti-Me cold agglutinin in these serological findings. However, the activity of this agglutinin against RBC was markedly decreased in the presence of human milk. The data suggest that the agglutinin is directed against a new glycolipid antigen common to human RBC, termed Om antigen.