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 less than 0.001), 0.84 (p less than 0.001), 0.94 (p less than 0.001) and 0.92 (p less than 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 less than 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.