The antibacterial activity of fresh and heat-inactivated normal serum was compared with that of sera from patients with renal failure: 16 on diet, 9 on regular hemodialysis (HD) and 9 on continuous ambulatory peritoneal dialysis (CAPD). The antibacterial activity was determined on Proteus rettgeri (Sanelli) by a turbidimetric method. The inhibitory activity of fresh serum was only slightly descreased in nondialyzed uremic patients, whereas it was significantly impaired in CAPD and HD patients. Heat-inactivated normal serum (56°C, 30 min) lost its antibacterial activity. Only CAPD patients’ sera behaved as the normal ones. In fact, a consistent residual antibacterial activity was found in heat-inactivated sera of nondialyzed and hemodialyzed patients. The results are in keeping with the view that uremic patients have reduced host-defence reactions. Dialysis treatment appears to further depress the antibacterial capacity of uremic patient’s sera. The residual antibacterial activity of heat-inactivated sera is due to substances, not present in normal controls, which act with a mechanism independent of the complement system. CAPD seems more effective than hemodialysis in removing these substa