Two cyclic AMP-independent casein kinases can be isolated from human erythrocyte hemolysate, one of which (referred to as 'casein kinase S') phosphorylates only serine residues of whole commercial casein, while the other (referred to as 'casein kinase TS') phosphorylates both serine and threonine residues of the same substrate. Moreover, the casein kinase S, unlike casein kinase TS, is able to phosphorylate the erythrocyte membrane proteins. The present paper deals with the further characterization of casein kinase S, freed from histone kinase activity by DEAE and subsequent phosphocellulose chromatography of the crude hemolysate in the presence of 0.2% Triton X-100. In particular, cytosol casein kinase S exhibits some physico-chemical and catalytic properties identical to those of the membrane-bound casein kinase, solubilised and purified as previously described. Both casein kinases display the same chromatographic behaviour, the same Sepharose elution volume, the same optimal pH range, the same Km for casein and ATP, the same response to NaCl, MgCl2 and CaCl2, and the same ability to phosphorylate serine but not threonine residues of beta-casein.