We have investigated the modifying influence of 17β-oestradiol on the anti-metabolic and growth inhibitory actions of methotrexate (MTX) in a human breast cancer cell line, MDA-MB-436. This cell line contains detectable oestrogen receptors but is progesterone receptor-negative. Furthermore, 17β-oestradiol (10-10-10-6 M) failed to influence DNA synthetic rate as assessed by [3H]-TdR or [3H]-UdR incorporation and cell proliferative rate was similarly unaffected. Although by these criteria 17β-oestradiol failed to elicit a biological response in the MDA-MB-436 cell line, 10-6M 17β-oestradiol significantly reduced the anti-metabolic and anti-proliferative actions of MTX. In the presence of 17β-oestradiol approximately twice the concentration of MTX was required to inhibit cell proliferation to the same extent as was observed following exposure to MTX alone. This partial reversal of MTX effects was accompanied by a 20% reduction in the steady-state intracellular MTX concentration when cells were exposed to the drug in the presence of 10-6 M 17β-oestradiol.