Suckling Swiss mice were injected daily for 8 days with either electrophoretically pure (EP) mouse interferon (s.a. 4.7 x 10(8) units/mg protein), major impurities obtained in the course of purification, or partially purified mouse interferon (s.a. 1.3 x 10(7) units/mg protein). Only EP or partially purified interferon inhibited growth, induced liver and kidney lesions, and killed mice. The authors conclude that interferon itself is responsible for these effects.