Passage of the virulent H37Rv strain of tubercle bacillus in normal or immune systems (normal or immune monocytes suspended in the corresponding serum) resulted in decreased virulence of the bacilli; this was evidenced by the very low mortality rates in mice inoculated intravenously with passaged bacilli. Passaged bacilli when cultivated directly in tween-albumin medium or when grown on glycerol-blood agar plates after recovery from infected mouse tissues proved as virulent as unpassaged bacilli. The decreased virulence of passaged H37Rv was accompanied by loss of ability to bind neutral red. Passaged H37Rv was more sensitive than unpassaged bacilli to inactivation by sodium oleate and by normal monocyte lysate; however, passaged H37Rv was more resistant than unpassaged bacilli to inhibition by streptomycin.
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