IGF-1 and 2 are thought to be important growth factors for breast cancer. However, gene expression of IGFs or IGF receptors in breast cancer tissues, and especially in metastatic breast cancer cells, is not well known. Expression of mRNA encoding for IGF-1, IGF-2, IGF-receptor 1 and 2, IGF binding proteins- 1 to -6, insulin receptor and insulin was determined in the NIH MCF-7 breast cancer cell line, in specimens from breast cancer tissues, and in 6 primary breast cancer cell cultures obtained from metastatic breast cancer, using rT-PCR technique. Specific mRNA sequences encoding for IGF-receptor 1 and 2, IGFBP-2, -4 and insulin receptor were identified in all cell cultures and most of the tissue specimens. Though in most of the tissues additional expression of IGF-1 and IGF-2 was detected, there was no mRNA encoding for these proteins in MCF-7 cell cultures as well as in the primary cell cultures of metastatic breast cancers. In none of our specimens mRNA encoding for IGFBP-1, -3, -5, -6 and insulin was detectable. IGF-receptor expression in cancer tissues and metastatic breast cancer cells supports the hypothesis that IGFs increase tumor cell proliferation in vivo. Expression of IGF-1 and IGF-2 in tumor tissues but not in cancer cell cultures indicates an IGF expression located predominantly in stromal parts of cancer tissues.