A heat-stable trichloracetic acid-stable protein fraction stimulates protein kinase activity in chick brain cytosol. This protein kinase (tentatively referred to as protein kinase S) can be partially purified by chromotography on DEAE-cellulose and Sepharose G-100. The partially purified protein kinase has an absolute requirement for magnesium and the heat-stable protein for the phosphotransferase activity and is not influenced by cyclic nucleotides, calcium or ethylenediaminetraacetic acid. The substrate specificity of protein kinase S indicates that it is not a casein kinase and prefers histones over the subsrtates tested. The specific activity of this protein kinase changes with chick brain development and the activity increased by two-fold by the second post-hatch week, suggesting a role of this protein kinase in chick brain development.