Several clinical protocols are attempting to utilize the combined anti-proliferative signal of interferon-alpha (INF-alpha) and tamoxifen on cancer cells. We demonstrated here that the effect of these two agents on the growth of the premacrophage U937 cells is antagonistic. This antagonistic effect is paralleled by the ability of tamoxifen to modulate the INF-alpha-induced hyperpolarization in these cells. INF-alpha-induced hyperpolarization was shown before to be an integral part of the anti-proliferative signal of this agent. Tamoxifen liberates Ca2+ from intracellular stores of U937 cells but this effect of the drug is not the cause of its antagonistic effect with the anti-proliferative signal of IFN-alpha. We suggest therefore, that the combined effect of these two anti-cancer drugs could also be advantageous for macrophage proliferation.