Interferon (IFN), typesβandγ, and IFN inducers polyinosinic‐polycytidylic acid and lymphocytic choriomeningitis virus all stimulated the generation of blast‐NK cells in mouse spleens. Blast‐NK cells were characterized on the basis of size,3H‐thymidine uptake, and NK cell markers. These data indicate that in addition to augmenting NK cell‐mediated lysis, IFN may regulate NK cell proliferation in vivo.