Human interleukin 1 (IL 1) exerts an interferon-like antiviral activity in certain strains of human diploid fibroblasts. It is shown that this antiviral effect is indirect, in that it is mediated by production of interferon-beta. IL 1 also induces synthesis of the mRNA of another secreted protein, 26K, whose biological function is still controversial. Finally, IL 1 has bone marrow colony stimulating activity which is shown to be due to induction of colony stimulating factor in adherent phagocytic cells present in the bone marrow cultures. These findings emphasize the concept that several of the varied biological effects of IL 1 may be indirect.