The humoral immune response against endogenous ecotropic murine leukemia viruses (MuLV) was examined in irradiated and control C57BL/6 mice. Control mice developed antibodies against MuLV slowly throughout life. In contrast, within 2-3 mo after irradiation 90% of irradiated C57BL/6 mice had developed detectable antibodies against MuLV. The characteristics of this immune response, however, were identical in control and irradiated mice in terms of peak titers, specificity for endogenous ecotropic MuLV, and reactivity against the ecotropic viruses' glycoprotein (gp71). Moreover, the rate of appearance of antibodies against MuLV in irradiated mice and the peak titers were generally not affected by age at irradiation, dose of irradiation (two, three, or four treatments of 175 R), or bone marrow reconstitution. Although the ability of irradiation to accelerate the appearance of antibody in a population of C57BL/6 mice suggested activation of endogenous ecotropic MuLV, there was no apparent correlation between the appearance of this immune response or its persistence and the development of lymphoma. Thus, the incidence of lymphoma was comparable in mice that: (a) developed no immune response; (b) developed an immune response only transiently after irradiation; or (c) developed an immune response which persisted until death from lymphoma. Moreover, experimental conditions that alter the ability of irradiation to induce leukemia, such as age, dose, or bone marrow reconstitution did so without significantly altering either the rate of appearance of a humoral immune response to MuLV or its peak titers. The results, therefore, fail to demonstrate any seroepidemological relationship between endogenous ecotropic MuLV and radiation-induced leukemia.