Production of vascular endothelial growth factor (VEGF) and expression of its receptors Flt-1 and KDR was determined in primary cultures of separated epithelial and stromal-enriched cultures derived from ten primary human breast carcinomas. By enzyme-linked immunosorbent assay, epithelial cells produced a mean VEGF of 33 +/- 7 pg ml(-1) microg(-1) RNA (range 11-70). Stromal cells produced similar levels, with a mean of 48 +/- 11 pg ml(-1) microg(-1) RNA (range 7-92). This was significantly greater than the amount produced by similar cultures derived from normal breast tissue (epithelial mean 19 +/- 5 pg ml(-1) microg(-1) RNA, range 9-34, P < 0.05 vs tumour epithelial culture; stromal mean 26 +/- 8 pg ml(-1) microg(-1) RNA, range 3-56). Flt-1 and KDR receptors were analysed by semi-quantitative reverse transcription polymerase chain reaction. Flt-1 was expressed by four of six epithelial and five of six stromal cultures. When expressed by both cell types, Flt-1 appeared to be significantly more abundant on stromal cells compared with epithelial cultures. Only a single tumour, a lobular carcinoma, failed to express Flt-1 on either cell type. With KDR, the reverse was true with constitutive expression of this receptor by epithelial cultures and zero or reduced (3/6) expression by stromal cultures. Differences in the expression pattern of VEGF receptors may reflect a differential response to VEGF by specific cell types. Thus, production of VEGF and expression of VEGF receptors Flt-1 and KDR by breast cancer epithelial and stromal cells suggests that VEGF may fulfil not only an angiogenic role, but also play a fundamental role as an autocrine/paracrine regulator in breast cancer, thereby facilitating tumour proliferation and subsequent invasion.