Recent studies have revealed altered regulation of cyclins in breast cancer cells. To understand the role of aberrant cyclin B1 expression in the proliferation of breast cancer cells, we examined cyclin B1-associated proteins in estrogen-responsive MCF-7 cells in a cell cycle-dependent manner. Immunoprecipitation of cell lysate with a monoclonal anti-human cyclin B1 antibody, followed by Western blot probing with an anti-human cdc2 (PSTAIR) antibody revealed the presence of a 34 kDa protein in estradiol-treated cells at 16 h after initiation of cell cycle progression. Flow cytometry and [H-3]-thymidine (Thd) incorporation experiments showed a dramatic increase in the percentage of S phase cells at this time point. This protein was suppressed by an antiestrogen, 4-hydroxytamoxifen. It was not found in MCF-1OA, a normal breast epithelial cell line. The 34 kDa protein was not reactive with antibodies raised against other cyclin dependent kinases (CDKs), including p34(cdc2(Carboxy terminal)). This protein was functionally active as determined by histone H1 kinase activity. These data suggest that the induction of a cyclin B1-associated 34 kDa protein during the G1 --> S transition might be a positive regulator of cell cycle progression in estrogen-responsive breast cancer cells.