Effects of combining anti-estrogens and R115,777 on the inhibition of MCF-7 cell proliferation. Studies were performed using 1.5 × 105 cells in 60 mm dishes incubated for 5 days with increasing concentrations of (a) R115,777 and tamoxifen, (b) R115,777 and ICI182,780, or (c) R115,777 and PBPE. Dose response interactions between these anti-estrogens and R115,777 were evaluated using the isobologram method. The isoeffect curves were constructed, at the IC50 point, according to the Materials and methods . X symbolizes the data point obtained with a combination of the two agents giving 50% inhibition. (a) When X falls within the envelope of additivity, the combination is considered as having an additive effect. (b,c) When X falls to the left of the envelope the combination is considered as having a supra-additive effect (synergism). PBPE, N-pyrrolidine(-phenylmethyl-phenoxy)-ethanamine-HCl.