Dendritic cells (DC) are professional antigen-presenting cells with a unique capacity to initiate and regulate immune responses. Immature CD1a⁺ DC can be cultured from CD14⁺monocytes in the presence of interleukin (IL)-4 and granulocyte macrophage colony-stimulating factor in vitro. Results of this study show that the nonsteroidal anti-estrogens toremifene and tamoxifen inhibit this differentiation. In the presence of anti-estrogens the cells lose CD14 expression, but remain CD1a⁻ and clearly have less dendritic processes than immature DC. Functionally, anti-estrogen-treated cells are inferior to immature DC in inducing proliferation of allogeneic T cells and in producing IL-12 p70 protein after CD40 ligation. The expression of the costimulatory molecules CD80 and CD86 is differentially regulated by anti-estrogens during DC differentiation. Furthermore, anti-estrogens are also able to inhibit the terminal maturation of DC. By inhibiting the functional differentiation of DC, anti-estrogens may have a role in the treatment and prevention of autoimmune diseases.