Endothelial 1 (E1) is identified as an endothelial cell secreted factor that stimulates collagen gel contraction by fibroblasts. This identification is based on (a) co-localization of stimulatory activity in endothelial cell conditioned media with synthetic E1 in reversed phase analysis; (b) removal of the activity from conditioned media with antiserum directed against E1; and (c) the activity of synthetic E1. Treatment of endothelial cell conditioned media with immobilized anti-E1 antibodies removed 59% of the activity from the pool suggesting that E1 is the major contraction promoter in endothelial cell conditioned medium. The mechanism of action of E1 is shown to be different from serum in that E1-promoted contraction is dependent upon the synthesis of an unknown effector protein. Synthetic E1 is shown to be a potent promoter of gel contraction with half-maximal activity occurring at 32 pM. Two other endothelins, E2 and VIC, are slightly less active than E1. A fourth endothelin species, E3, is substantially less active. A comparison of E1 with other contraction promoting peptides revealed that E1 and platelet-derived growth factor are essentially equal in specific activity, whereas TGF beta is approximately 50-fold more potent.