The effects of serine phosphorylation on the DNA cleavage/religation equilibrium of topoisomerase II and the sensitivity of the enzyme to antineoplastic drugs were characterized. Both casein kinase II and protein kinase C were used for these studies. Each kinase incorporated a maximum of approximately 1.4 phosphate molecules per homodimer of topoisomerase II. When the enzyme was incubated with both kinases simultaneously, phosphate incorporation increased to approximately 2.6 molecules/homodimer. In the absence of antineoplastic drugs, phosphorylation had only a slight effect on the DNA cleavage/religation equilibrium of topoisomerase II. However, in the presence of etoposide or 4'-(9-acridinylamino)methane-sulfon-m-anisidide, phosphorylation attenuated the ability of drugs to stabilize enzyme-DNA cleavage complexes. Levels of drug-induced DNA cleavage products decreased approximately 33% following phosphorylation of topoisomerase II by casein kinase II, approximately 17% following modification by protein kinase C, and approximately 50% following simultaneous phosphorylation of the enzyme by both kinases. This latter 50% reduction in DNA cleavage products correlated with an approximately 2-fold increase in the apparent first order rate constant for DNA religation mediated by simultaneously modified topoisomerase II. These results strongly suggest that the sensitivity of topoisomerase II toward antineoplastic drugs can be modulated by altering the phosphorylation state of the enzyme.