Increasing numbers of HIV-infected patients who have ESRD are being treated with continuous ambulatory peritoneal dialysis (CAPD). To investigate the potential infectious nature of peritoneal dialysate (PD), the peritoneal dialysis effluent was studied in 14 patients on CAPD who were known to be HIV antibody positive. Peripheral blood mononuclear cells and the sediment of PD were obtained from each patient and subjected to a qualitative microculture assay using a coculture of patient cells or PD fluid with peripheral blood mononuclear cells from non-HIV-infected individuals. Samples from the coculture were collected twice weekly for HIV P24 antigen determination as a marker of viral replication. PD, white blood cell and red blood cell counts, and peripheral blood CD4 lymphocyte counts were also measured. All 14 patients developed a positive blood culture by Day 3. Twelve of the 14 patients developed a positive PD fluid culture. The mean CD4 count was 310 cells/mm3. No patient had clinical or cellular evidence of peritonitis at the time of fluid sampling. These data indicate that peritoneal dialysis effluent from patients who are HIV antibody positive is potentially infectious.