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1. The results of light microscopical studies using Y. enterocolitica-strains grown at room temperature or at 37 degrees C, respectively indicated, that at the higher temperature cell mechanisms which are involved in the division process were hampered. This observation provided a basis for the fact that cultures of Y. enterocolitica incubated at 37 degrees C yield less cells than those grown at room temperature for which an example is given (Fig. 1). Additionally it was seen that the cells grown at room temperature had a smaller volume than those incubated at 37 degrees C (Table 3). 2. It is a well-known fact that Y. enterocolitica expresses some factors which are supposed to increase or to establish the pathogenicity for the natural host merely or only when cultivated at room temperature. It will be shown that this statement is not true when the pathogenicity is studied in white mice and those strains are tested, which owe their letality after i.p. inoculation to the content of endotoxin. The LD50 of the strains cultivated at different temperatures was very similar, i.e. about 1 X 10 9 cells (Table 4). Only when the comparison was made on a dryweight basis the toxicity as well as the room temperature were used (Table 1 and 2). The latter results can be attributed to the greater amount of cell wall material present in equal masses of cells due to the smaller seize of the cells grown at room temperature.