Inflammation of the rat footpad followed injection of cryoglobulin in crystalline form (Type I) and injection of cryoglobulin in solution (Type II). Rats deficient in essential fatty acids responded with diminished swelling which corrected to normal levels by addition of prostaglandin E1 suggesting that this reaction is prostaglandin mediated. Addition of bradykinin produced no effect. Aggregated cryoglobulin proved more inflammogenic than non-aggregated cryoglobulin. Pre-treatment with choline salicylate and colchicine reduced swelling while pre-treatment with dipyridamole increased edema following cryoglobulin inoculation. Cryoglobulin is considered to be an acute phase reactant in inflammation.