The administration of a potent activator of macrophages (M phi), Propionibacterium acnes, in nondiabetic mice was associated with the release of significant amounts of interleukin-1 (IL-1) in the peritoneal cavity and plasma within 4 hours after treatment. Shortly before IL-1 peaks were observed, the levels of pancreatic insulin, [3H]leucine-proinsulin, and insulin/total protein ratio were elevated, and followed by a transient but marked hyperinsulinemia at 4 hours after treatment. A single dose of recombinant murine IL-1 in mice was also associated with a 2- to 9-fold increase in the levels of insulin in the pancreas and plasma at 4 hours after treatment. During the period of observation after the administration of P. acnes, plasma glucose levels in treated mice were significantly less than in parallel controls. Mild hypoglycemia was observed at 7 to 10 days posttreatment. Although circulating IL-1-like activity could not be detected in plasma 1 to 10 days after P. acnes treatment, this activity was measured in activated peritoneal and liver M phi. IL-1-like activity (specific activity: 276 units/mg protein) was detected in plasma, after it was chromatographed on a Sephadex G-150 column to remove proteins with higher molecular weight. Peritoneal and liver M phi from P. acnes mice were also able to elaborate significant amounts of IL-1-like activity in their supernatants with or without Escherichia coli lipopolysaccharide. At the same time, total protein synthesis and insulin content in the pancreas in P. acnes mice were significantly lower than the parallel control (p less than 0.01). These results suggest that P. acnes-induced M phi activation in mice was associated with the modulation of insulin release and glucose homeostasis which may be attributed to the accumulation and release of IL-1 by activated M phi.