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.

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