Normal non-immune calf, human and mice sera kill-preferentially the Shigella flexneri 1b serotype cells with antigenic formula I; 3, 4, 6 leaving the antigenic mutant cells of the serotype 3b with antigenic formula III; 3, 4, 6. The strongest lethal action was observed in normal calf serum. This serum makes possible to obtain the 3b serotypes mutant from pure population of 1b serotype cells. Thermal inactivation of the complement deprives the serum of its selective activity.
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