Casein kinase II is a ubiquitous serine-threonine kinase made of two different (alpha and beta) subunits, with an alpha 2 beta 2 stoichiometry, alpha bearing the catalytic site of the enzyme. Two different genes encode two different (alpha and alpha') catalytic subunits. The present work shows that at least two forms of CKII (CKIIa and CKIIb) can be isolated from bovine tissues and adrenocortical cells in culture. The use of specific antibodies developed against the alpha and alpha' subunits disclosed that while CKIIa contains both alpha and alpha', the alpha subunit was highly prominent in CKIIb. This suggests that CKIIb may be an alpha 2 beta 2 oligomer whereas CKIIa may be an alpha alpha' beta 2 moiety or an alpha alpha' beta 2 and alpha' 2 beta 2 mixture. Transition from quiescence to proliferation was concomitant with an average two fold increase in total cell CKII activity involving both CKIIa and CKIIb isoforms. The two isoforms were found in cytosolic and particulate adrenocortical subcellular preparations and they similarly increased their association to intracellular organelles in response to growth stimulatory conditions.