To define the role of humorally mediated antimicrobial activity of human ascitic fluid, bactericidal assays as well as quantitation of complement components (C'3 and C'4), total hemolytic complement, immunoglobulins (IgG, A, and M), and lysozyme were performed on cell-free ascitic fluid from 7 patients with Laennec's cirrhosis. Assays using Escherichia coli and Klebsiella pneumoniae showed bactericidal activity for both organisms in four fluids and bacteriostatic activity for E. coli in one. All seven fluids were bacteriostatic for Bacteroides fragilis, however, no inhibitory activity was seen for either Streptococcus pneumoniae or Streptococcus faecalis. The effect of heat, zymosan, and ethylene glycol bis(β-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) was also tested on three bactericidal specimens. Heat and zymosan abolished bactericidal activity; in contrast, EGTA had no effect. Bactericidal fluids had significantly higher C'3 concentrations (mean ± sem, 29.7 ± 2.4 mg per dl versus 15.3 ± 2.2 mg per dl, P