In all, 40 major polypeptides ranging in molecular weights from 14.5 to 83 kDa were shown to be induced by IFNs alpha (also by IFN-alpha 2b and beta in a few cases) and gamma in human cultured cells of epithelial (transformed amnion cells (AMA)), fibroblast (proliferating and quiescent MRC-5 fibroblasts), and lymphoid origin (Molt-4). With the exception of a heat shock protein (IEF14 or hs x 70) and two tropomyosins (IEFs 52x and 55), none of these proteins corresponded to polypeptides (proliferation-sensitive or others) previously identified and catalogued by us. IFN-alpha induced the highest number of polypeptides in lymphoid cells, while the response to IFN-gamma was more pronounced in cultured epithelial and fibroblast cells. Several of the polypeptides induced by IFNs alpha and gamma were synthesized (albeit at different rates) by the control untreated cells, and in some cell types such as normal human peripheral blood mononuclear cells many were expressed at high levels. Only IFN-alpha-induced a unique set of proteins (alpha 1, 51 kDa; alpha 2, 15 kDa; alpha 19, 78 kDa; and gamma 10, 83 kDa) in all cultured cell types studied, implying that response to this IFN involves a shared biochemical pathway(s). Both IFN-alpha (also IFN-alpha 2b) and beta induced an identical group of proteins in AMA cells in agreement with the fact that type I IFNs share common receptors. IFNs alpha and gamma induced a few common polypeptides, but only gamma 10 (83 kDa) showed increased synthesis in all cell types exposed to either of these IFNs. A total of 28 major cellular polypeptides were down-regulated by IFNs in the various cell type studied. Different sets of proteins were affected, however, in each system, emphasizing the complexity of the mechanisms underlying the action of these factors. Treatment of synchronized G1 AMA cells with IFNs alpha, beta, or gamma (500 IU/ml, final concentration) did not inhibit their progression from G1 to S-phase as determined by indirect immunofluorescence using PCNA autoantibodies specific for cyclin. These observations were in line with the fact that IFNs did not affect dividin or cyclin(PCNA) synthesis (S-phase specific proteins) at least within the first 17 hr after their addition.