The primary anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) antibody response is known to have a heteroclitic fine specificity, i.e., anti-NP antibodies bind (4-hydroxy-5-iodo-3-nitrophenyl)acetyl (NIP) with greater affinity than NP itself. Past studies of NP-specific DTH responses and NP-specific T cell-mediated suppression have demonstrated sharing of fine specificity patterns and idiotypic structure between receptors on NP-specific T cells and anti-NP antibodies. We now analyze the fine specificity of NP-specific cutaneous sensitivity (CS) reactions to NP-O-succinimide (NP-O-Su) and NIP-O-succinimide (NIP-O-Su). The specificity of these responses is shown to be controlled by genes in the Igh gene complex. Cross-reactive CS responses induced by NP-O-Su elicited by NIP-O-Su were observed in strains of mice possessing the Igh-1b allotype but not in strains bearing the Igh-1c or Igh-1j allotypes. The CS reactivity could be adoptively transferred to naive recipients, and the ability of transfer CS reactivity was T cell dependent. In contrast to the genetic requirement for I-A region homology to adoptively transfer DTH reactions, compatibility at either the H-2K, H-21, or H-2D regions was sufficient to transfer NP-specific CS reactivity to naive recipients. Furthermore, in contrast to DTH responses, cyclophosphamide pretreatment was not required to induce CS responsiveness. Thus, the specificity of NP-O-Su-induced CS responses is controlled by both H-2- and Igh-linked genes.