Serum lysozyme and hemolytic complement (CH100) levels were measured in dogs with experimental Bacteroides peritonitis. The CH100 levels showed little change in surviving animals. Nonsurvivors showed a moderate decrease in complement levels shortly after contamination. Both surviving and nonsurviving animals showed a slight initial decrease in lysozyme levels shortly after contamination. In surviving dogs this was followed by an increase to normal levels. In nonsurvivors, levels continued to increase, reaching a threefold magnification just prior to death. As a result of antibiotic therapy, CH100 levels exhibited no major changes; however, dogs deprived of antibiotic showed noticeable and persistent increases in lysozyme levels while treated animals showed only a mild elevation in lysozyme levels. The changes in the level of serum lysozyme may be a good indicator of antibiotic efficacy and approaching death from Bacteroides peritonitis.