The experiments reported here indicate that, when exposed to insulin, viable lymphocytes rapidly released into the incubation medium a factor capable of increasing the dextran-induced anaphylactoid reaction, but having no effect on the inflammatory response evoked by 5-HT. This pro-inflammatory factor was shown to be elaborated by cell suspensions derived from lymph nodes of rats, rabbits, pigs or calves as well as from human tonsils. Thymus cells showed no such activity. The pro-inflammatory factor was termed as anaphylactoid-inflammation-promoting factor (AIPF). Its production depended upon the dose of insulin, and the time of exposure. AIPF was found to have an elution pattern in Sephadex G-100 gels similar to that of BSA (67,000 daltons). The activity was abolished by heat or incubation with DNase or a-chymotrypsin, but was not influenced by RNase. AIPF by itself did not induce increased vascular permeability, and proved to be distinct from the permeability factors present in the lysate of lymph node cells.