Endothelial cells can be induced to form a branching network of tubular structures using a variety of cell culture conditions. We have examined this differentiation process using several sets of conditions: plating human umbilical vein endothelial cells (HUVEC) on Matrigel, adding collagen to the apical surface of HUVEC grown on fibronectin, and plating HUVEC on fibrin in the presence of FGF-1. We determined that although the first two conditions produce dramatic morphologic changes in the HUVEC population, gene transcription and translation are not required for the regulation of the process. Rather, post-translational events are involved since the Matrigel-dependent process could be inhibited by the addition of nocodazole, suramin or H7, a protein kinase inhibitor. In contrast, the fibrin matrix-dependent differentiation pathway involved transcriptional and translational events since the addition of either actinomycin D or cycloheximide inhibited this pathway. A modified differential display of RNA extracted from HUVEC after 0, 2, 5, and 24 hours on fibrin revealed expression of a novel cDNA.