The yeast Cdc7 protein is indispensable to initiation of nuclear DNA replication, based on the phenotype of the conditional, temperature-sensitive (ts) cdc7 mutants at the restrictive temperature. This protein has likewise been implicated in commitment to meiotic DNA recombination and induced mutagenesis, which may result from error-prone DNA repair. Our previous work revealed sequence similarity between the Cdc7 protein and known protein kinases. To determine whether it possesses kinase activity, we have immunoprecipitated the protein from Cdc7-overproducing yeast cells by using polyclonal antibodies raised against a nondenatured beta-galactosidase-Cdc7 fusion protein. In this report, we demonstrate that Cdc7 immune complexes are capable of phosphorylating mammalian histone H1 on serine and/or threonine residues. Immune complexes derived from cells harboring the cdc7-2 ts mutant gene on a high copy number plasmid possess a thermolabile kinase activity. Thus, we postulate that Cdc7 may regulate the various DNA metabolic pathways by phosphorylating one or more target substrates. Because Cdc7 kinase acts downstream of Cdc28/cdc2 kinase function at "start," the transition from G1 to S phase in the cell cycle may be the result of a cascade of protein phosphorylation.