Initiation factor 2 from adult rat brain was isolated from salt-washed microsomes using a three-step purification process consisting of heparin-Sepharose, phosphocellulose and diethylaminoethyl-cellulose (DEAE cellulose) column chromatographies. The initiation factor 2(eIF-2) was phosphorylated in subunits alpha and beta by the endogenous protein kinase activity present in the pruified preparation. This protein kinase activity proved to be mostly a casein kinase, although the possible presence of a very specific alpha kinase activity cannot be dismissed.