The antibody Fl shows preferential reactivity with adult erythrocytes over newborn erythrocytes, and its reactivity is abolished by sialidase treatment of the erythrocyte. The antibody was found to recognize binary determinants linked to the branched lacto-N-isooctaosylceramide (formula; see text) The presence of an N-acetylneuraminyl group at one end and L-fucosyl group at the other end is essential for the reactivity of the antibody. A substitution at the penultimate D-galactosyl residue of one of the chains with an alpha-D-(1 leads to 3)-linked 2-acetamido-2-deoxygalactosyl or galactosyl group did not inhibit the reactivity of the antibody. The new blood group A- and B-active, branched gangliosides are also isolated and characterized.