Cyclosporine A (an immunosuppressive cyclic undercapeptide) acts as an antagonist to prolactin receptors on a breast tumour cell line (MCF7) which is known to express specific prolactin receptors. The competition between cyclosporine A and prolactin to prolactin receptors was demonstrated by measuring the decreased specific 125I-labeled prolactin-binding to prolactin receptors in the presence of increasing concentrations of cyclosporine A. Moreover, cyclosporine A inhibiting effects on the cell growth were investigated in this mammary cancer (MCF) by measuring the cell DNA content with flow cytometry. Our results should prove very useful in understanding the mechanisms of prolactin regulation of DNA synthesis in tumorigenesis of the mammary gland.