Antibodies to the somatic antigens of Gram-negative bacilli have been demonstrated to occur in three classes of immunoglobulins (1-10). Identification of specific antibody has depended upon the association of immune activity with immunoglobulin-rich fractions prepared from whole serum by various physical and chemical techniques, Immune activity in these fractions has been measured b y examining secondary properties of antigen-antibody interaction such as agglutination of particulate antigens or particulate carriers coated with antigen, preparation of live bacteria for phagocytosis, and complement-dependent bactericidal effect. The results of such experiments have led to the tentative conclusion that all or most of the immune proteins of the serum specific to the somatic antigens of Gram-negative bacilli are of the ~,M-class of immunoglobulins (3,5,(8)(9)(10). I n the studies described below the secondary activities of highly purified rabbit ~M-and ~/G-antibodies to the somatic antigens of Salmonella typhirauriura were examined and were found to differ markedly from each other.Methods and Materials 1. Preparation of the Antigen.--Salmondla typkimurium, strain 7 x (4, 5, 12:z) Salmonella paratypM B 2 (4, 5, 12:b), and Escherichia coli were grown in brain heart infusion broth (Difco Laboratories, Inc., Detroit) with constant agitation for 15 hours at 37°C. The "O" or somatic antigen was prepared as described (12). The heat-killed whole cell antigens used for experiments involving absorption and/or absorption-elution were utilized within 15 hours after preparation. Antigen preparations for assay of agglutination and for immunization were stored at 4°C without preservative. 2.Immunization.--Adult hybrid rabbits were injected with 1 X 101° organisms of S.

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A technique has been described for the cultivation in vitro of normal mononuclear cells on glass slides in a liquid medium. Under these conditions the monocytes transformed into macrophages which proliferated as in ordinary tissue culture. These cultures of monocytes could be infected with tubercle bacilli. The numbers of stainable tubercle bacilli within the monocytes increased steadily in cultures infected with virulent or attenuated strains. Evidence is given to support the view that this increase in numbers of bacilli was due to intracellular multiplication. There was no evidence of intracellular bacillary multiplication in cultures infected with an avirulent strain. Tubercle bacilli multiplying within phagocytes in vitro exert a damaging effect upon the host cells. The damage was most obvious in cells infected with a virulent strain. Tubercle bacilli within phagocytes were protected against the bacteriostatic effect of streptomycin added in a concentration of 5 γ per ml. of culture medium. This permitted the use of streptomycin in infected cultures to prevent extracellular multiplication of the bacilli.