Abstract Vitamin D3, cholecalciferol, induces cell—cell communication via gap junctions in murine fibroblasts (C3H/10T 12 cells) at concentrations between 0.01 and 1.0 μM, as assayed by the dye transfer method using Lucifer yellow CH. The extent of induction is similar to that obtained with the positive controls, canthaxanthin or retinoic acid, applied at 10 and 1 μM, respectively. Vitamin D2 also induces cell—cell communication. At elevated concentrations of vitamin D3 (5 μM) there is a suppression of gap junctional communication, reversible upon exposure to all-trans retinoic acid (1 μM) after removal of vitamin D3 from the medium. Conversely, communication between cells prestimulated with retinoic acid (1 μM) rapidly decreases when the retinoid is replaced by vitamin D3 (5 μM). The data demonstrate a role for vitamin D in the regulation of intercellular communication. This novel property of vitamin D may contribute to the antiproliferative effects of vitamin D exhibited in some types of cancer.