Peritoneal cells were isolated from dialysates drained from nine patients on continuous ambulatory peritoneal dialysis (CAPD) during episodes of peritonitis. Levels of expression of mRNA for the regulatory cytokines, interferon-gamma (IFN-gamma) and IL-4, were investigated daily, where possible, during the first 5 days of peritonitis. Cytokine mRNA levels were compared with those of peripheral blood mononuclear cells (PBMC) stimulated in vitro with phorbol 12-myristate 13-acetate (PMA) plus phytohaemagglutinin (PHA). Peritoneal cells expressed low levels of IFN-gamma mRNA; for four of nine patients, IL-4 mRNA levels greater than those expressed by stimulated PBMC were detected. There was no pattern of cytokine mRNA expression associated with the types of organisms detected in dialysates at initiation of peritonitis. However, in contrast to those patients with a transient, resolving peritonitis, significant IL-4 mRNA expression was detected in cells isolated early in the episodes of peritonitis in patients who suffered recurrent peritonitis within 30 days of the initial peritonitis episode. These results suggest an association between early IL-4 mRNA expression and susceptibility to further infections. The known anti-inflammatory effects of IL-4 may explain this association.