Summary Absorption studies with human anti-A and anti-B sera suggested the presence of B-like antigen on marmoset red blood cells and the absence of any A-like antigen. Reaction of specific anti-A and anti-B elu-ates with marmoset cells confirmed the absorption tests. Eluates prepared from the absorption of the anti-A and anti-B reagents with marmoset red cells suggested, however, the presence of an A-like antigen in addition to the B-like antigen on their red cells. Inhibition tests on marmoset saliva suggested the presence of the A antigen and the absence of B and H antigenic substances in their saliva. Addendum. In current iso- and heteroim-munization studies aimed at obtaining reagents capable of differentiating marmosets by their red cell antigens, the presence of anti-B agglutinins (log2 titer range 1-5) to human B cells has been detected in the sera of 18 of 30 marmosets tested. Weaker reactions to human A cells have been found in two. The presence of the anti-B agglutinins in their sera correlates with the A-like substance on the marmoset red cells and in the saliva, and suggests further the non-identity of the B-like substance found on their cells to that on human B cells. Wiener et al(5) have noted the lack of any anti-A agglutinins in 11 marmoset sera tested and the presence of anti-B agglutinins in four.