The antiproliferative effect of interferon was studied at various stages of pre- and postimplantation mouse embryo development. Although 3.4 .times. 103 reference units/ml of NDV [Newcastle disease virus] induced mouse interferon did not inhibit developmnet of DNA synthesis in preimplantation embryos, an inhibitory effect was first detectable in primary cell cultures derived from whole 8-day embryos. This effect reached a maximum of 36-44% inhibition of DNA synthesis in cells cultured from 10 day embryos and remained at the same level with cells cultured from 12, 14 and 17 day embryos. Responsiveness to interferon is achieved in mouse embryos by 10 days.