Casein kinase II (CK II), a key enzyme involved in the regulation of cell growth, has been variously reported to be activated by diverse mitogens, including insulin-like growth factor 1 (IGF-1) and epidermal growth factor (EGF). Activation of the enzyme is generally carried out in the presence of serum, and we examined the question whether serum components participate in the activation process. We demonstrated previously that ML-1 cells require IGF-1 plus transferrin (TF) for growth and transforming growth factor beta or tumor necrosis factor alpha plus TF for differentiation. We now found that CK II is activated only when the cells are exposed to both IGF-1 and TF or when TF is replaced in this combination with relatively high levels of iron salts. Induction of differentiation with TGF-beta and TF did not result in CK II activation. These results show that CK II activation in ML-1 cells requires the application of both components of the growth signal, IGF-1 and TF, demonstrating that the growth factor alone is incapable of enhancing the activity of the enzyme.