Treatment of MCF7 human mammary carcinoma cells with the nonsteroidal antioestrogens, tamoxifen and clomiphene, leads to a concentration-dependent decrease in cellular proliferation rate which can be resolved into oestrogen-reversible and oestrogen-irreversible components. This became more clearly apparent when cells were treated with the 4-hydroxylated derivatives of these compounds where, because of enhanced affinity for the oestrogen receptor (ER), the dose-response curves for the two components could be separated. Thus treatment with 4-hydroxyclomiphene resulted in a distinct biphasic effect on cell growth. In the concentration range 10(-10)-10(-8) M, cell proliferation was inhibited in a concentration-dependent manner to a maximum of 60-70%, there was no further effect between 10(-8) and 10(-6) M, but at concentrations greater than 10(-6) M there was another concentration-dependent decrease in cell growth. Studies with a series of vinyl-substituted hydroxytriphenylethylenes revealed that in the nanomolar concentration range, where the effects of the drugs could be completely negated by the simultaneous addition of oestradiol, the potency for growth inhibition was highly correlated with affinity for ER. Such data provide strong evidence that in this concentration range the growth inhibitory effects of nonsteroidal antioestrogens are mediated by the intracellular ER. In the micromolar concentration range the effects of antioestrogens are not completely reversed by oestradiol, potency is not well correlated with affinity for either ER or the antioestrogen binding site (AEBS) but the effect is cell cycle phase-specific. Furthermore, the disparity between the affinity for AEBS (0.8-3.3 nM) and the concentration of drug needed for oestrogen-irreversible growth inhibition (greater than or equal to 2.5 microM) argue against a central role for AEBS in mediating this effect. The observation that triphenylethylene antioestrogens are calmodulin antagonists may provide some insight into potential mechanisms for this oestrogen-irreversible effect. Indeed, in identical experiments two phenothiazine calmodulin antagonists inhibited MCF 7 cell proliferation at concentrations greater than or equal to 2.5 x 10(-6) M. Growth inhibition following administration of fluphenazine, perphenazine and triphenylethylene antioestrogens was accompanied by qualitatively similar changes in the cell cycle kinetic parameters, i.e. accumulation in G1 phase at the expense of S phase cells. These data suggest triphenylethylene antagonism of calmodulin activated cellular processes as a potential mechanism for the oestrogen-irreversible effects of the nonsteroidal antioestrogens.