C5a, released during activation of the complement cascade, was measured by radioimmunoassay in human plasma incubated with Dacron, preclotted Dacron, glutaraldehyde treated human umbilical vein (HUV), polytetrafluoroethylene (PTFE), and Dacron-collagen composite vascular grafts. Expressed as percent of C5a in control plasma incubated without graft, C5a generation by preclotted Dacron, and by the HUV was similar to that by Dacron (941 +/- 206% S.E.M.), while that due to PTFE was markedly less (p = 0.005). The Dacron-collagen composite vascular graft also generated significantly less C5a than Dacron and was similar to PTFE in this respect. These results expand on previous work suggesting that lower C5a generation by PTFE explained the negligible polymorphonuclear infiltrate seen on its surface in vivo, allowing it to endothelialise as rapidly as Dacron despite poorer attachment of seeded endothelial cells. The role of complement as a factor limiting endothelialisation of synthetic vascular prostheses needs further investigation.