During the past two years our laboratory has isolated Propionibacterium-an anaerobic Gram-positive rod-on 40 occasions from the drainage fluid of 25 patients on CAPD and eight on IPD. Most patients had no symptoms and the fluid was sent for culture after treatment of peritonitis, during training period, and occasionally because it was cloudy. No association was found with patients' age, length on dialysis, or previous peritonitis episodes. The only statistically significant association (P = 0.033) was with recent (less than 30 days) catheter insertion. Patients with Propionibacterium as the only organism in their dialysis fluid had normal leucocyte count in the effluent, minimal or no symptoms, and received no treatment. In order to establish the frequency of positive cultures of asymptomatic patients, we cultured 95 dialysate effluents from 33 asymptomatic CAPD patients. Cultures were observed for 4 weeks. We found seven positive fluids (7.4%) in four patients (12%), aerobic sporeformer (2), Staphylococcus epidermidis (2) and Streptococcus viridans (1). The mean time to grow was 18.6 days. In conclusion, effluents from asymptomatic CAPD patients may contain bacteria. The organisms are commensals requiring prolonged incubation to grow. Propionibacterium isolated from peritoneal effluent of patients with minimal or no symptoms does not require treatment. These findings suggest that errors in the technique of CAPD are not rare but when the number of organisms is small and the organism has low virulence, peritonitis does not occur.