Previous electron-microscopic studies of isolated islets of Langerhans exposed to the monokine interleukin-1 for 7 days have indicated that interleukin-1 is cytotoxic to all islet cells. To study the time-course and possible cellular specificity of interleukin-1 cytotoxicity to islets exposed to interleukin-1 for short time periods, isolated rat or human islets were incubated with or without 25 U/ml highly purified human interleukin-1 for 24 h. Samples of rat islets were taken after 5 min, 30 min, 1, 2, 4, 6, 8, 10, 12, 16, 20 and 24 h and samples of human islets after 5 min, 30 min and 24 h of incubation and examined by electron microscopy in a blinded fashion. Already after 30 min, accumulation of opaque intracytoplasmic bodies without apparent surrounding membranes, and autophagic vacuoles were seen in about 20% of the beta cells examined in rat islets exposed to interleukin-1. After 16 h of incubation with interleukin-1, more than 80% of rat beta cells showed signs of degeneration. Beta cell specific changes similar to those observed in rat islets exposed to IL-1 for 30 min were seen in human islets exposed to IL-1 for 24 h. The described changes were not observed in alpha cells in interleukin-1-treated rat or human islets, or in alpha and beta cells in control islets. Passing interleukin-1 over columns containing Sepharose-coupled anti-interleukin-1 antibody completely removed the beta cell cytotoxic action on rat islets.(ABSTRACT TRUNCATED AT 250 WORDS)