Eggs incubated at 40°C. developed only minimal rickettsial infection, whereas control eggs incubated at 37.5°C. became heavily infected. Potassium cyanide markedly enhanced rickettsial growth in experiments in which the control eggs developed only minimal infection. Under circumstances such that the control eggs became heavily infected, KCN had no appreciable effect.

Toluidin blue and methylene blue delayed the development of rickettsial infection in the yolk sac, but their rickettsiostatic action under the conditions of these experiments was less marked than that of penicillin and para-aminobenzoic acid.

The rickettsiostatic action resulting from temperature elevation was neutralized by KCN, and hence is believed to be due to the increased activity of the cyanide-sensitive respiratory enzyme (cytochrome oxidase) in the entodermal cells in which the rickettsiae multiply.

The rickettsiostatic action of toluidin blue and methylene blue, though probably also resulting from increased metabolic activity in the entodermal cells, was not neutralized by KCN. This observation is in harmony with the reported observation that dyes of this type furnish an alternative mechanism for intracellular oxidation which is cyanide-insensitive.

The rickettsiostatic action of para-aminobenzoic acid was not neutralized by KCN. No conclusions can be reached at present concerning the mechanism of action of this compound.

Dinitrophenol and several compounds related to para-aminobenzoic acid gave negative results.