Hormonal and environmental factors that control the growth, differentiation, and regression of the vasculature are of fundamental importance in tumorigenesis and in the choice of therapeutic strategies. To test the hypothesis that estradiol (E 2 ) and basement membrane proteins would affect the survival of vascular endothelial cells (EC), immortalized human umbilical vein endothelial cells (ECV304) were examined for their response to the chemotherapeutic drugs taxol and etoposide. ECV cell apoptosis was inhibited by E 2 (taxol only) or attachment to extracellular matrix (ECM) (taxol or etoposide). E 2 increased ECV growth, while ECM binding resulted in growth arrest and differentiation. Apoptosis was associated with decreased levels of Bcl-2 and p21 proteins. E 2 prevented downregulation of p21 and Bcl-2 induced by taxol but did not prevent the down-regulation of p21 induced by etoposide, consistent with the failure of E 2 to inhibit etoposide-induced cell death. However, ECM prevented p21 and Bcl-2 down-regulation induced by taxol or etoposide. Persistent activation of NFB occurred after attachment of ECV cells to ECM, suggesting a role in survival or differentiation. IB␣ levels were not affected by taxol but were reduced by etoposide treatment, while IB levels did not change with drug treatment. E 2 did not alter the levels of IB␣ or IB. Interestingly, levels of IB␣ and IB declined in etoposide-treated ECV cells on ECM concomitant with the elevation of NFB, suggesting that in these cells degradation of IB may be responsible for NFB activation. In agreement with these data, anti-sense NFB treatment of ECV cells inhibited differentiation on ECM, but did not affect cell survival. In conclusion, culture of ECV cells on ECM or treatment with E 2 inhibited apoptosis. NFB activation by ECM was necessary for cellular differentiation, rather than inhibition, of apoptosis.