Summary: Hapten-specific delayed hypersensitivity of guinea pigs was suppressed by a variety of hapten-amino acid and hapten-protein conjugates. Suppression was short-lived when the conjugate carried one, two or three arsanilate groups but lasted longer than 11 days when larger carrier molecules with more arsanilate groups were injected. These results were interpreted as meaning that larger conjugates reacted with and used up the supply of reactive moiety in delayed sensitivity, while smaller conjugates were only inhibitory by virtue of reversible binding with the same entity.