A rat model was developed to determine if delayed hypersensitivity skin tests during early peritonitis would predict outcome. Presensitized rats were simultaneously tested intradermally with keyhole-limpet hemocyanin and given four types of fecal bacteria and 10% barium sulfate intraperitoneally. Rats were divided into four groups according to inoculum dose. Skin tests were read at 24 hours in survivors and correlated with death and abscesses during the next 19 days. In the two groups with greatest mortality, 35% of anergic rats died, compared with 0% of reactive rats. In the three groups with most abscesses, anergic rats had more abscesses than those that reacted. Overall, 90% of anergic rats died or had abscesses v only 10% of reactors. We concluded that the outcome of untreated peritonitis is determined in the first 24 hours; anergy at that time predicts death or abscess formation.