Members of the p56 family of mammalian proteins are strongly induced in virus-infected cells and in cells treated with interferons or double-stranded RNA. Previously, we have reported that human p56 inhibits initiation of translation by binding to the "e" subunit of eukaryotic initiation factor 3 (eIF3) and subsequently interfering with the eIF3/eIF2.GTP.Met-tRNAi (ternary complex) interaction. Here we report that mouse p56 also interferes with eIF3 functions and inhibits translation. However, the murine protein binds to the "c" subunit, not the "e" subunit, of eIF3. Consequently, it has only a marginal effect on eIF3.ternary complex interaction. Instead, the major inhibitory effect of mouse p56 is manifested at a different step of translation initiation, namely the binding of eIF4F to the 40 S ribosomal subunit.eIF3.ternary complex. Thus, mouse and human p56 proteins block different functions of eIF3 by binding to its different subunits.