Since patients on continuous ambulatory peritoneal dialysis are at high risk for peritonitis, the opsonins in peritoneal dialysis effluent responsible for phagocytosis and a neutrophil chemiluminescence response to surface-adherent Staphylococcus epidermidis were examined. In surface phagocytosis assays uninfected dialysate was as opsonic as 1% serum. The opsonic activity was heat stable and equal to that of purified IgG at the same concentration (0.1 mg/ml). In contrast, optimal chemiluminescence to surface-adherent Staphylococcus epidermidis was dependent on complement. C3 deposition on Staphylococcus epidermidis opsonized in dialysate was quantitated by an enzyme immunoassay (EIA) and represented 13% of control C3 deposited with opsonization in 10% serum. Unused dialysate was found to be inhibitory to neutrophil phagocytosis and complement deposition. A combination of the low pH and high dextrose concentration of dialysate was responsible, but restoration of the pH to 7.4 largely restored both indices. During peritonitis there was a parallel increase in IgG levels and C3 deposition (r = 0.8), and surface phagocytosis was also enhanced. On further analysis, subjects with a single episode of peritonitis had a significantly more opsonic peritoneal effluent than those who had two infections during the study. This latter group had a poor IgG response to infection. This study demonstrates the relative deficiencies of host defence in the peritoneal cavity and indicates that measures to improve opsonin delivery and reduce the inhibitory effects of dialysate would be beneficial.