At 41°C. the organisms were not found in the cultures histologically on and after the 2nd day in vitro, and the cultures were non-virulent on and after the 3rd day in vitro, in spite of good preservation and growth of the cells.

At 37.5°C. organisms were absent from the cultures histologically and the cultures were non-virulent on and after the 11th day in vitro, in spite of good preservation and growth of the cells.

At 32°C. good but slow growth of cells occurred and organisms were found in increasing numbers histologically up to about the 21st day in vitro. At this time, 50 to 99 per cent (approximately) of the cytoplasmic volume of the cultures was occupied by Rickettsiae. From the 21st day to the 51st day (the limit to which cultures have been carried successfully) this condition of unrestricted multiplication remained practically unchanged. Inoculation of guinea pigs with single cultures after varying lengths of time in vitro, (up to the 51st day) always resulted in reproduction of typhus in a characteristic manner.

At 27° the cell growth was negligible, but many cells remained alive for 10 days or more. Organisms were absent from the cultures histologically and the cultures were non-virulent on and after the 18th day in vitro.

The only histological preparations showing unrestricted multiplication of the organisms (infection of the majority of the cells present) were of cultures incubated at 32°C.

It is believed that the detrimental effect of the higher temperatures (37.5° and 41°C.) on the multiplication of the organism is exerted indirectly, by stimulation of the defence mechanism of the cells.