For determining the mechanism of genetic control of D-penicillamine free-base-specific delayed-type hypersensitivity (DTH) in mice, footpad swelling response was employed. Studies with various congenic and recombinant inbred strains of mice revealed that D-penicillamine free-base-specific DTH was controlled by the I-A subregion. The footpad swelling response showed a highly antigen-specific pattern, and revealed that D-penicillamine free base was not cross-reactive with D-penicillamine disulfide or L-penicillamine, but with D-penicillamine HCl. Treatment of immune lymphoid cells with monoclonal antibodies plus complement revealed that the cells responsible for DTH transfer were Lyt-1+2-, L3T4+ and Ia-T cells.