Injections of zymosan mixed with egg-white and Freund adjuvant, into rabbits and guinea pigs, resulted in increased bactericidal activity against Escherichia coli B. This activity was maintained for at least one month. The animals responded to intracutaneous injections of zymosan with the Arthus phenomenon. It is suggested that the properdin fraction of serum contains the normal serum substances which destroy bacteria and neutralize viruses, and that the injection of zymosan stimulates the production of antibodies similar to those found in the properdin of normal serum.