The present authors have studied the reticulum cells of the spleens in bacterial infectious cases with the silver impregnation method which was improved for the purpose of investigating reticuloendothelial cells. On the impregnated sections, the splenic reticulum cells were divided into the following four types; (1) branching, (2) polymorphic, (3) round, and (4) foamy. The polymorphic and round cells were considered to be branching types activated or mobilized. The polymorphic and round cells increased in the spleens of the infectious cases, except in those cases showing exhaustion of vital reaction.