Pure recombinant human IL-6 inhibits growth of T47D, MCF-7 and SK-BR-3 human breast carcinoma and OVCAR-3 ovarian carcinoma cells in cultures. Subcloning of the breast ductal carcinoma T47D cells yields clones with high and low sensitivity to the growth inhibitory effect of IL-6. The subclones vary 40 fold in their sensitivity for inhibition of colony formation in sparse cultures and 200 fold for inhibition of thymidine incorporation in subconfluent cultures. Binding studies with 125I-rIL6 show that T47D cells and their subclones, as well as SK-BR-3 and MCF-7 cells, express high-affinity receptors for IL-6. The number and affinity constant of these receptors are comparable to those on lymphocytic and myeloid cells, and show no correlation with the high or low sensitivity phenotype. Proliferation of the breast cancer cells is inhibited by IL-6 in a cell density dependent manner, and is not due to a cytotoxic effect. In addition, IL-6 induces a morphological change with loss of epithelial characteristics and of cell-cell adhesion. Sensitivity to growth inhibition by IL-6 is independent from that of IFN-beta 1, IFN-gamma or TNF.