Four days after i.v. immunization of mice with optimal low doses of heterologous erythrocytes (2 x 10(5) RBC), strong delayed-type hypersensitivity (DTH) responses can be elicited in the footpad. At later intervals after immunization, DTH responsiveness is progressively diminished and replaced by 4-hr antibody-dependent reactions. These evanescent T cell-mediated DTH responses, which are progressively replaced by antibody-dependent reactions, resemble Jones-Mote type delayed hypersensitivity responses of humans and guinea pigs. Since higher doses of immunizing antigen activate suppressor mechanisms that inhibit DTH responses, we examined the possibility that the evanescence of DTH in mice immunized with an optimal low dose of antigen might also be due to suppression. Using techniques that could clearly demonstrate the suppression produced by high antigen doses, we failed to find evidence for either humoral or cellular suppression in optimally immunized mice with declining of DTH responses. Thus, it appears that the evanescence of produced by optimal low dose immunization with RBC may be due to an intrinsic short life span of the effector cells rather than to the activation of an identifiable shut-off mechanism.