The multiplication of Friend leukemia cells in the spleens of irradiated or nonirradiated histocompatible DBA/2 mice was determined by the uptake of 125-5-iodo-2'-deoxyuridine. The multiplication was inhibited by daily treatment with mouse interferon preparations or the interferon inducer, Newcastle disease virus. Since, under the experimental conditions used, the proliferation of the Friend leukemia cells was independent of the replication of the Friend virus, the inhibitory effect of interferon might not be mediated by an antiviral effect but by some other mechanism.