Eukaryotic protein synthesis initiation factor 2 (eIF-2) can be phosphorylated on its tl subunit by two wellcharacterised protein kinases, termed the haem-controlled repressor (HCR) and the double-stranded RNAactivated inhibitor (dsI). Phosphorylation of eIF-2 by these kinases is thought to be important in the regulation of peptide-chain initiation. We report the location of the serine residue in the a subunit, which is phosphorylated by both these enzymes. Limited tryptic digestion and subsequent cyanogen bromide treatment of rat liver eIF-2 phosphorylated by HCR yielded one major phosphopeptide. This peptide had the sequence Ile-Leu-Leu-Ser-Glu-Leu-Ser(P)-Arg-Arg .The same major phosphopeptide was obtained from rabbit reticulocyte eIF-2 phosphorylated by HCR or dsl as judged by its behaviour on two-dimensional mapping and reverse-phase chromatography. In all cases the phosphorylated residue was found to be serine-7, and not serine-4, of the above sequence as determined from sequence analysis and by subdigestion of the peptide with Staphylococcus aureus V8 proteinase.Protein synthesis initiation factor-2 (eIF-2) mediates the binding of the initiator tRNA (Met-tRNAi) to the 40s ribosomal subunit during peptide-chain initiation. The activity of eIF-2 is believed to be important in controlling the overall rate of chain initiation and is regulated by phosphorylation of its smallest (a) subunit (reviewed in [l-31). Briefly, phosphorylation of eIF-2a impairs the recycling of eIF-2 which is required to regenerate active eIF-2 from the inactive [eIF-2 . GDP] complexes which are released from the ribosome after each round of chain initiation [4]. This recycling is mediated by an additional initiation factor [5 -91 termed GEF (guanine-nucleotide-exchange factor). This factor promotes the exchange of bound GDP for GTP and yields the active [eIF-2 . GTP] species, which can bind the initiator Met-tRNA, and participate in a further round of chain initiation.The role of the phosphorylation of eIF-2a in the control of translation has been studied extensively in reticulocytes, where haem deprivation rapidly leads to inhibition of chain initiation associated with increased phosphorylation of eIF-2a. This results from the activation of a haem-regulated