Certain complement inhibitors, namely chlorpromazine, suramin, 2-hydroxystilbamidine and chlorophenothiazine sulphonate were tested for their ability to suppress complement deposition and vascular injury at the site of an Arthus reaction. Deposition of complement was suppressed in the order 2-hydroxystilbamidine greater than suramin greater than chlorpromazine. All the above mentioned four compounds strongly protected vascular injury as observed by electron microscopic studies. At Arthus reaction sites prepared without drug treatment venules ranged from normal to severely altered and damaged. Discontinuities in endothelial linings varied from small to longer stretches. In the latter situation remaining endothelial cells were degenerated and endothelial remnants did not have an intact basal lamina. After treatment with the above complement inhibitors, at arthus reaction sites some venules appeared normal, whereas others were altered but in all cases the endothelium and its basal lamina remained intact.