Scanning immune electron microscopy using a monoclonal anti-A antibody which reacts with all type A oligosaccharide chains revealed A antigens on less than 5% of A(m) and A(el) cells, some of which showed extremely strong labelling. This explains why A(m) and A(el) cells can absorb significant amounts of anti-A without being agglutinated. A(3) may be a heterogenous subgroup, since A antigens were found on 82 and 58%, respectively, of the cells of 2 A(3) individuals. A antigens were found on 75% or more of A(x) cells. In many weak A individuals A-positive cells are apparently best detected if an anti-A is used which reacts strongly with other A oligosaccharide chains than type 2. From hyperimmune pregnancy sera A(x), A(m) and A(el) erythrocytes absorbed antibodies which seemed to have other fine specificities than those absorbed by A(2) cells. We conclude that weak subgroups of A may deviate from A2 both by number of erythrocytes expressing A antigens and the biochemical nature of the antigens.