Direct in vitro effects of IL-1 alpha on cell cycle progression of the estrogen-responsive, MCF-7, and estrogen-unresponsive, MDA-231, human breast cancer cells were investigated by flow cytometry. IL-1 alpha, at nanomolar concentrations, caused the synchronization of MCF-7, but not MDA-231, cells in the G0/G1 phase of the cell cycle. The S phase of IL-1 treated MCF-7 cells was correspondingly decreased. The IL-1 induced synchronization of MCF-7 cells was observed in the dose range of 10(-11) M to 10(-7) M and was seen as early as 6 h after the start of treatment. Furthermore, these effects were shown to be sensitive to the weak estrogen, phenol red, since the IL-1-induced shifts in G0/G1 and S phases were markedly blunted in its presence. In cell proliferation experiments, the IL-1-induced synchronization of MCF-7 cells increased the cytotoxic efficacy of the chemotherapeutic drug, 5-fluorodeoxyuracil. These data demonstrate that IL-1 by arresting the estrogen-responsive human breast cancer cells, MCF-7, in the G0/G1-phase of the cell cycle can not only directly inhibit the growth of MCF-7 cells, but also increase the efficacy of FUDR.