Swiss white mice were immunized with different mycobacteria and delayed type hypersensitivity (DTH) responses were studied by the foot-pad swelling technique of Gray and Jennings (1955). Extensive cross-reactions in DTH, outside the limits of Runyon's groups were observed. As a general trend slow growing mycobacteria showed greater cross-reactivity with slow growers than with rapid growers and vice versa. The implied cross-protective significance of DTH cross-reactions was further confirmed by demonstration of the ability of DTH cross-reacting sonicates to generate activated macrophages in M. avium immunized mice. An antiserum was raised against the earlier reported DTH eliciting antigen of M. tuberculosis H37Rv (DTH-H37Rv). The sero-reactivity of anti-DTH-H37Rv against the sonicates of different mycobacteria was studied with the objective of investigating the molecular basis of DTH cross-reactivity. Immunoprecipitation reactions of different mycobacterial sonicates with anti-DTH-H37Rv showed that the antigen was shared by all the mycobacteria tested irrespective of their cross-reactivity in a DTH response. All of the slow growers showed reactions of total identity with DTH-H37Rv. However with rapid growers DTH-H37Rv showed only a partial identity. From these data it was concluded that an antigen participating in DTH response is shared by all mycobacteria and that it is polymorphous, having genus specific and group specific (as slow and rapid grower groups) determinants.