In a prospective study of intraperitoneal opsonins in 30 patients undergoing continuous ambulatory peritoneal dialysis (CAPD), the IgG concentration, the fibronectin concentration, the specific antistaphylococcal antibody level, and the opsonic activity against Staphylococcus epidermidis were measured in peritoneal dialysis effluent from the initiation of CAPD and monthly for 6 months. Significant correlation was found between the four assays, but the interindividual and intraindividual variations were considerable. No statistically significant correlation was observed between susceptibility of the patients to CAPD-related infectious peritonitis and any of the above-mentioned parameters of humoral defense. We conclude that at the present time it is not feasible to use these assays for the establishment of prognosis with regard to peritonitis in CAPD.