Effects of some media used for suspending rickettsiae during purification, for metabolic studies, and in titrations of infectious rickettsiae were examined with respect to the plaque-forming ability of Rickettsia rickettsi and R. typhi in primary chicken embryo tissue cultures and the infectivity of R. typhi in mice. Brain heart infusion broth (BHI) was found superior to all other media tested in preventing both a significant decrease in plaque-forming units (PFU) and a delay in plaque formation. Skim milk, egg yolk, and some metabolic media were effective in maintaining PFU at 0 C, but did not prevent a significant delay in plaque formation. However, infectivity of R. typhi for tissue culture and mice was markedly decreased when suspended in metabolic media at 26 C. Addition of BHI to the routine tissue culture overlay reversed the deleterious effects of sucrose-phosphate solutions. The effects of Mg(2+), Mn(2+), K(+), Na(+), sucrose, and glutamate were also examined. No significant differences were observed between R. rickettsi and R. typhi in their responses to different media. The results of this study suggest the necessity for a reappraisal of previous studies of metabolism and infectivity of rickettsiae in these media.