Mouse polyclonal antibodies, raised against three regions of the human BRCA1 protein, were characterized and revealed BRCA1 as a 220-kDa nuclear phosphoprotein in normal cells. All three antisera recognize both in vitro-translated and recombinant, baculovirus-derived BRCA1, which co-migrate with BRCA1 from the human breast epithelia cell line, HBL100. BRCA1 expression and phosphorylation are shown to be cell cycle dependent, with greatest expression and phosphorylation occurring in S and M phases. Cyclin-dependent kinase 2 and other kinases associated with cyclins D and A are shown to bind to and phosphorylate BRCA1, suggesting that the biological activity of BRCA1 may be regulated by cyclin-dependent kinases.