A soluble extract from rat spleen, spleen permeability factor (SPF), predominantly protein in content, increases vascular permeability in rat skin by release of histamine and 5-HT from tissue stores. Increase in vascular permeability by SPF is markedly reduced by pretreatment with a combination ofmepyramine and methysergide or by local depletion of skin tissue amines by48/80. SPF releases histamine and 5-HT from rat peritoneal mast cells. This differs from histamine release by antigen, 48/80 and octylamine as it is blocked by 0.1 mM iodoacetic acid and is unaffected by calcium lack or by preheating the cells to 45~ for 5 minutes. There is no detectable kinin release by SPF which is devoid of general proteolytic or TAMe esterolytic activity at neutral pH. Nor does it seem to contain prostaglandins. The possible importance of SPF in the inflammatory response in delayed hypersensitivity reactions is discussed.