Abstract Endothelial cells from bovine aorta, vena cava, and cornea secrete a novel collagen in vitro (Sage et al., 1980). Endothelial collagen (EC), which is sensitive to pepsin and to several neutral proteases, exhibited an additional unusual property in its mode of secretion. In the absence of added sodium ascorbate, EC was secreted by both aortic and corneal endothelial cells at levels which were very similar to those observed in cultures supplemented with this vitamin. In contrast, the secretion of type III procollagen, which normally constitutes 75-80 0/o of collagenous protein in the culture medium, was significantly decreased in ascorbate-deficient cultures. Incubation of aortic endothelial cells with a, a'-dipyridyl, an inhibitor of prolyl and lysyl hydroxylases, reduced the extent of prolyl hydroxylation in total culture medium protein by 98 0/o but also did not affect the secretion of EC. The secretion of EC by endothelial cells appears to be independent of a requirement for prolyl hydroxylation. This property differs markedly from the secretory characteristics of the interstitial procollagens and more closely resembles that described for type IV (basement membrane) procollagen.