Cdc7 is a serine-threonine kinase which regulates initiation and progression of DNA replication. The activity of purified Cdc7 kinase is significantly stimulated by polyamines such as spermine or spermidine. Positively charged polymers of lysine or arginine also stimulate its kinase activity, whereas the negatively charged substances such as polyglutamate or nucleic acids significantly inhibit the kinase activity. Spermine affects both Km and Vmax of Cdc7 kinase for a MCM substrate. We also found that histones, lysine- and arginine-rich basic protein, can stimulate Cdc7 kinase activity, and a MCM complex, in association with Histone, is more efficient substrate of Cdc7 than the free MCM complex. These results identify potential cellular inhibitors and stimulators of Cdc7 kinase and suggest that Cdc7 may be another target of cellular polyamines and that histones may stimulate Cdc7-mediated phosphorylation of chromatin-bound substrates. Ectopic expression of an antizyme, known to reduce the cellular polyamine level, resulted in reduction of Cdc7-mediated phosphorylation of MCM4 protein, suggesting physiological roles of polyamines in regulation of Cdc7 kiase activity in the cells.