The sensitizing properties of unsaturated and halogenated sultones were examined in a guinea-pig model for contact hypersensitivity. The sensitization potential of 2-chloro-1,3-dodecanesultone was similar to that of 1-chloro-2,4-dinitrobenzene (DNCB). 2-bromo-1,3-dodecanesultone, 1-decene-1,3-sultone, 1-decene-1,3-sultone, 1-tetradecene-1,3-sultone and 1-hexadecene-1,3-sultone were stronger senistizers than DNCB. Ectensive cross-reactivity was observed among sultones of the same side-chain length, but differing from each other in being either unsaturated or halogenated. Extensive cross-reactivity was also noted among 4 unsaturated sultones of differing side-chain lengths. The inter-relationships among these sultones in regard to their immunogenic and tolerogenic properties are discussed.