The opsonic activity of 60 ascitic fluids from 47 patients was measured using a standard opsonophagocytic assay. Curve analysis of the opsonic activity compared to the ascitic fluid concentration of total protein, total hemolytic complement, C3 and C4 yielded correlation coefficients of 0.84 (p < 0.001), 0.84 (p < 0.001), 0.94 (p < 0.001) and 0.92 (p < 0.001), respectively. There appeared to be a threshold of concentration for each protein below which there was no killing of bacteria. Cirrhotic ascites had significantly (all p < 0.001) lower concentrations of total protein and complement and less opsonic activity than noncirrhotic ascites (including malignant, cardiac and miscellaneous types). Perhaps it is the dilution of crucial antimicrobial proteins below a threshold which predisposes to spontaneous bacterial peritonitis.