In this study we investigated the role of interleukin 1 (IL 1) in the induction of inflammatory lesions and in the preparation and provocation of the local Shwartzman reaction. Both of these phenomena can be induced with a variety of agents. This suggested to us that a common endogenous mediator may be crucial to the development of these two lesions. When IL 1 was injected intradermally into shaved rabbit backs, 51Cr-labeled neutrophils accumulated at the injection site. Neutrophils began to accumulate less than 1 hr after injection, and the maximum rate of accumulation was observed by 4 hr. This activity was dose dependent. It was calculated that in all animals, 10(-14) mol of IL 1 induced significant neutrophil accumulation, whereas in many animals, as little as 10(-15) mol of IL 1 sufficed. When 4.2 X 10(-9) mol of E. coli 0111:B4 lipopolysaccharide W was injected i.v. 24 hr after an intradermal injection of IL 1 (2.9 X 10(-13) mol), a local Shwartzman reaction was seen 4 hr later at the intradermal injection site. IL 1 injected i.v. 24 hr after an intradermal injection of either IL 1 or lipopolysaccharide also produced a local Shwartzman reaction. These data indicate that IL 1 may be the common endogenous mediator of the inflammatory response, and IL 1 may serve in the same role for the preparation and provocation of the local Shwartzman reaction.