The effective concentrations of penicillin in vitro and in vivo for streptococci, pneumococci, and Treponema pallidum.

In the therapeutic use of penicillin, the question as to whether the minimum effective concentration in vivo is the same, greater, or less than that necessary to kill the same organ'sm in vitro is of obvious importance. Data are here reported with respect to the minimal effective serum concentrations of penicillin in a number of experimental infections in mice and rabbits. As will be shown, these serum levels were usually 2 to 5 times higher than those necessary to kill the same organism in vitro. However, in view of the known concentration differential between the serum and tissue fluids, this suggests that the effective concentration of penicillin at the actual site of infection is in most instances the same as that effective in vitro.