Minute amounts of the anti‐L3T4 antibody designated GK1.5 were found to deeply suppress in vivo antibody responses to T‐dependent antigens. Primary responses to sheep erythrocytes were completely inhibited even when GK1.5 was administered up to 6 days before or 4 days after the antigen. Secondary responses to potent immunogens like sheep erythrocytes or keyhole limpet hemocyanin were also completely abolished by a single injection of GK1.5 just before the boost. This treatment had no effect on T‐independent reactions such a s the polyclonal activation of B lymphocytes with lipopolysaccharide or the anti‐2, 4,6‐ trinitrophenyl (TNP) response elicited by injection of TNP‐Ficoll.