Epidermal growth factor (EGF) stimulates surfactant protein A (SP-A) synthesis in human fetal lung explants. EGF receptor (EGF-R) binding leads to an increase in intrinsic tyrosine kinase activity with subsequent activation of second messengers. We hypothesized that inhibition of EGF-R tyrosine kinase activity would block surfactant protein A expression in spontaneously differentiating cultured human fetal lung tissue. We measured levels of SP-A protein after exposure to genistein, a preferential inhibitor of EGF-R tyrosine kinase activity (IC50=1.0 μM, 0.3 μg/ml). Midtrimester human fetal lung explants were prepared by mincing lung tissue after removal of the major airways and maintained in serum-free Waymouth's medium with and without genistein (1.0 - 10 μg/ml) for 4 days. Levels of SP-A protein were determined by immunoblot analysis. We found a significant 2-fold dose-dependent decrease in SP-A (ANOVA, n=4, p<0.001, Bonferroni p value< 0.05 at a genistein conc. > 5.0 μg/ml) with increasing concentrations of genistein. Genistein did not affect cell viability as measured by the release of lactate dehydrogenase nor affect the ultrastructural morphology of type II cells (lamellar bodies). Furthermore, genistein (10 μg/ml) blocked EGF-induced (10 ng/ml) increase of SP-A in fetal lung organ culture. We conclude that inhibition of tyrosine kinase with genistein blocks baseline and EGF-stimulated EGF-R activity resulting in a decrease in the level of SP-A in human fetal lung. Furthermore, tyrosine kinase inhibition did not block morphologic differentiation of type II cells implying a specific effect on SP-A rather than a global effect on fetal lung development. We speculate that EGF plays an important role in the regulation of human SP-A through activation of a tyrosine kinase dependent signal transduction pathway.