Four human breast cancer cell lines (T47D, ZR-75-1, MCF7D and HS578T) were examined for the effects of cytokines on expression of cell surface antigens. Interferon (IFN)gamma up-regulated the expression of ICAM-I (CD54) in all the cell lines and coordinately up-regulated both CD54 and CD40 expression in T47D. Tumour necrosis factor (TNF)alpha, interleukin (IL)1 alpha, IL1 beta and IL6 also up-regulated the expression of CD54 in all the cell lines but CD40 was unaffected. Levels of expression of CD11a, CD18, CD49b, CD58 and CD71 were unaltered by these cytokines. Conditioned medium (CM) generated from human fibroblasts, and in particular from foetal cells, was highly effective in up-regulating expression of ICAM-1 but not of CD40 in the breast cancer cell lines. ICAM-1 induction correlated with IL6 bioactivity in these CMs. Combinations of IL6 with other cytokines, such as IL1, resulted in further increases in ICAM-1 expression. Our observations suggest that IL6 is involved in intercellular signalling between mesenchyme and breast cancer epithelium.