MCF-7 human breast carcinoma cultures grown in the presence of 17-beta estradiol form solid, multicellular nodules, a process that reflects changes in cell-substrate adhesions and loss of growth inhibition. We examined the effects of estradiol on the status of tyrosine phosphorylation in focal adhesion kinase (FAK) and the association of FAK with paxillin using immunoprecipitations and then probing western blots for FAK, phosphotyrosine, and paxillin. Culture of MCF-7 cells for seven days in the presence of 1 nM E2 resulted in decreased tyrosine phosphorylation of FAK compared to controls. The estradiol-induced effect was blocked by 100 nM of the estrogen receptor antagonist 4-hydroxytamoxifen, indicating dephosphorylation of FAK is an estrogen receptor-mediated event. FAK immunoprecipitated from either estradiol or DMSO-treated cells phosphorylated the exogenous substrate poly(Glu,Tyr), suggesting that the potential kinase activity of FAK was not changed by estradiol. Estradiol treatment also resulted in a reduced association between FAK and paxillin. The decreased phosphorylation levels and reduced association between FAK and paxillin may be important steps leading to the loss of stable focal contacts and loss of growth inhibition during MCF-7 nodulation.