In an article entitled "Further Studies on Virulent Pseudo-diphtheria Bacilli" published two years ago,1 Dr. Horton and I described the properties of the serum of animals immunized against the " Ruediger bacillus," an organism belonging probably to the pseudo-diphtheria group. The serum of rabbits and of goats which had been immunized against the bacillus presented certain peculiarities. It contained specific agglutinin, opsonin, and bacteriolysin for these bacilli and showed itself extraordinarily resistant to heat, age, light, and drying. After the serum had been partially or wholly inactivated by heat, it was found to be impossible to reactivate it by adding fresh complement. Experiments in removing amboceptor by absorption also failed, and the bactericidal substance apparently did not consist of complement and amboceptor. The opsonin was also found to be highly resistant to heat and attempts to reactivate heated serum failed, but opsonin and bacteriolysin were apparently not the same substance. The conclusions drawn at the end of the work were as follows: