Mice injected with Propionibacterium acnes, when challenged with lipopolysaccharide release a range of soluble mediators into their serum. Included among these is lymphocyte activating factor (LAF, interleukin-1). The release of LAF in vivo was detected only when serum samples were assayed at high dilution because inhibitors of its activity in vitro were also present. The kinetics of release of LAF in vivo after injection of P. acnes was dependent on the mouse strain used. LAF was also detected in serum collected from nude mice, implying that the LAF activity measured in vitro was not due to contamination with T cell products, such as interleukin 2.