The properties of bioregulatory substances associated with embryogenesis and fetal development are under investigation. Growth stimulatory, as well as anti-proliferative and anti-viral factors, have been solubilized from mouse placental tissues taken after the 12th day of gestation, with primary emphasis on tissues taken at term. A rapid reliable photometric microassay was developed to quantitate murine interferon (IFN) utilizing an EIA reader. Aqueous placental extracts contained an acid-sensitive anti-viral activity which was specific for mouse cells. Under conditions of relaxed stringency, this IFN could be neutralized by antiserum to mouse fibroblast IFN, but not by antiserum to mouse immune IFN. The same placental extracts contained a trypsin-sensitive growth factor (PIGF) active in cultured fibroblasts. Anchorage independent growth of mouse and rat cells, mitogenesis of quiescent mouse and rat cells, and sustained replication of mouse and hamster cells was induced by PIGF. In contradistinction to epidermal and tumor growth factors, PIGF was stable in thiol reagent, and unstable at pH 3 and 80 degrees. The mitogenic activity was inhibited in the presence of mouse fibroblast IFN, as reported for other protein growth factors.