Two distinct mitogenic subcomponents of phytohemagglutinin (PHA)--leucoagglutinin (LA) and "purified" PHA--apparently stimulated different subpopulations of murine T cells. In the DBA/2J strain, the mitogenic responses of splenic lymphocytes to LA reached maximal levels after 24 to 36 hr exposure and almost completely disappeared by 48 hr, whereas maximal responses to PHA were maintained after 48 hr incubation. The levels of LA-responding T cells were highest in DBA/2J spleens at 5 weeks of age but markedly declined by 9 weeks of age, whereas thymic levels of LA-responding T cells reached a maximum at 9 weeks of age and remained maximal past 15 weeks of age. PHA-responding cells, in contrast, reached maximal levels in both the spleen and thymus of DBA/2J mice at 9 weeks of age. In C57BL/6J mice, splenic and thymic lymphocytes responded similarly to both components, except that the response of splenic lymphocytes to PHA reached a maximum after shorter incubation time and declined sooner than their response to LA. The mitogenic responses of C57BL/6J thymocytes to both components were already at their peak by 5 weeks of age and almost totally disappeared by 9 weeks, whereas the responses of splenic lymphocytes were maximal at 9-15 weeks of age. The responses of DBA/2J splenocytes to LA was significantly augmented by PHA, but LA markedly suppressed the proliferative responses to PHA.