The MCF-7 cell line is a hormone-responsive human breast-cancer cell line, which has been extensively used in studies of estrogen regulation of cell growth. These studies have indicated that the growth stimulation of the MCF-7 cells by estrogens may be effected by an autocrine mechanism involving several growth factors, such as EGF, TGF alpha and IGF-I and their receptors. We have amplified and cloned tyrosine-kinase-related sequences from the MCF-7 cell mRNA using the polymerase chain reaction and characterized the partial cDNAs obtained by nucleic acid sequencing. Nine tyrosine kinase cDNAs and one serine/threonine kinase cDNA were identified among the amplified sequences. Four different tyrosine kinase genes encoding receptors for fibroblast growth factors (FGFs) were found to be expressed by the MCF-7 cells. In addition, differences were observed in the expression of these members of FGF receptor family in different breast-cancer cells. A putative tyrosine-kinase receptor and a novel serine/threonine kinase were preferentially expressed in estrogen-responsive tumor cell lines. However, no estrogen-dependent regulation of any of the novel tyrosine-kinase receptor mRNAs was found in any of the cell lines including the MCF-7 or ZR-75-I cells, where the expression of the neu proto-oncogene mRNA was decreased during estrogen treatment. The expression of several FGF receptors by breast-cancer cells suggests that FGFs may be involved in their growth regulation and tumorigenesis.