In most human breast cancer cell lines, insulin, via its own receptor, stimulates cell growth. However, in MDA-MB231 breast cancer cells, insulin at concentration as high as 100 nM has no effect on cell growth, although insulin receptors (IRs) are overexpressed in these cells (29.1 ng IR/10(6) cells), and IR binding characteristics are similar to other breast cancer cell lines. IR tyrosine kinase activity is markedly reduced both in intact MDA-MB231 cells and in isolated IRs purified on a wheat germ agglutinin affinity column. MDA-MB231 cells contain a factor that inhibits both basal and insulin-stimulated IR tyrosine kinase activity in a concentration-dependent manner. This inhibitory activity copurifies with the IR on insulin-Sepharose affinity chromatography and is also effective against the tyrosine kinase activity of the IR-related insulin-like growth factor-I receptor and the oncoprotein v-abl but is ineffective against c-src tyrosine kinase activity. It is possible, therefore, that this tyrosine kinase inhibitor plays a role in regulating the mitogenic potential of the IR in some human breast cancers.