1. Among eight strains or variants, included in four different immunological types of pleuropneumonia-like microorganisms, all grew on chorioallantoic membranes; those belonging to Klieneberger's Type L5 (Sabin Type A) grew very poorly; and those included in three other types grew with varying degrees of vigor.

2. In all instances, strains of pleuropneumonia-like microorganisms tested grew better on dead sterile chorioallantoic membranes than on living membranes.

3. None of the strains tested was in itself lethal for chick embryos.

4. No constant macroscopic lesion developed as a result of inoculating chorioallantoic membranes with pleuropneumonia-like microorganisms.

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