Preparations of murine recombinant interferon (Mu-rIFN)-alpha, -beta and -gamma were assessed for their influence on in vitro growth of mast cells from normal mouse spleen cells (Sp C). Mast cell growth was inhibited by Mu-rIFNs when Sp C were exposed throughout the entire culture period to Mu-rIFNs. The most potent inhibitor of mast cell growth was Mu-rIFN-gamma, followed by Mu-rIFN-beta; Mu-rlFN-alpha had little effect. When added to IC-2 cells, clonal mast cell progenitor, both Mu-rlFN-beta and -gamma), significantly inhibited proliferative response of the target cells. The suppressive effect of Mu-rIFNs on IC-2 cells was selectively abolished by monoclonal antibodies against Mu-rIFN-beta and -gamma.