The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and four novel synthetic analogues (EB1089, KH1060, KH1230 and CB1093) on IGF-I-stimulated growth of MCF-7 human breast cancer cells have been determined. A significant time- and dose-dependent inhibition of IGF-I-stimulated cell growth was seen with EB1089, such that after 7 days of treatment with 10(-8) M EB1089, the mitogenic effect of IGF-I (30 ng/ml) was negated. Comparison with 1,25(OH)2D3 showed the synthetic analogues to be more potent. The anti-oestrogen ICI 182,780 similarly inhibited IGF-I-stimulated growth of these cells and in combination with EB1089 exerted additional inhibitory effects. Retinoids (all-trans-retinoic acid or the isomer 9-cis-retinoic acid) were less effective in limiting MCF-7 cell responsiveness to IGF-I but, in combination with EB1089, a co-operative effect was achieved. Using radioligand-binding techniques, we observed that 1,25(OH)2D3 and EB1089 down-regulated the levels of 125I-IGF-I binding to MCF-7 cell membranes. Scatchard analysis showed that EB1089 decreased maximal binding approximately 2-fold. Vitamin D derivatives were also demonstrated to reduce IGF-I receptor expression in MCF-7 cells by Western analysis. Our findings demonstrate that vitamin D derivatives limit responsiveness of MCF-7 cells to the mitogenic effects of IGF-I, which may be mediated by reduction of IGF-I receptor expression.