Four-week-old female AKR/J mice were given oral doses of powdered leaves from Echinacea purpurea three times weekly for 8 weeks (7.5 mg/mouse/week): controls received phosphate-buffered saline. Mean survival age of experimental AKR/J mice treated with the E. purpurea preparation was significantly prolonged and enlargement of thymic lymphoma in experimental mice was significantly suppressed compared with controls. In normal 3-week-old female AKR/J mice, mortality from thymic lymphoma was delayed markedly after injection into the thymus of cell-free extract of thymus from the experimental 28-week-old female AKR/J mice that received the oral E. purpurea preparation was injected directly into the thymus. Proliferation of endogenous recombinant murine leukemia viruses (MuLV) in the thymus was markedly inhibited after the first oral administration of the E. purpurea preparation as compared with untreated controls (final age, 28 weeks). Production of endogenous interferon (IFN)-gamma in AKR/J mice was also effectively augmented by the oral treatment with the E. purpurea preparation, however, the production of other cytokines such as tumor necrosis factor (TNF)-alpha and interleukin (IL)-12 was minimal. These results suggest that this suppressive effects on spontaneously occurring leukemia caused by endogenous recombinant MuLV in female AKR/J mice may depend on enhancement of nonspecific immune or cellular immune systems (or of both) by the E. purpurea preparation.