1. (a) Mammalian cells in tissue culture (mouse fibroblasts and malignant human uterine epithelium) did not concentrate penicillin from the culture medium. Even at low concentrations, the cellular accumulation was usually less than that in the surrounding fluid, and most of it was removed by washing. The radioactive material in such eluates was actively bactericidal, and was presumably in large part unchanged penicillin.
(b) Penicilloic acid, produced by the action of penicillinase, was bound to the same (limited) extent as the active antibiotic. In both these respects mammalian cells behaved like naturally penicillin-resistant bacteria, and unlike such penicillin-sensitive bacteria as Streptococcus pyogenes or Diplococcus pneumoniae.
2. Cell-free sonic extracts of the L strain had the same limited reactivity with penicillin as the intact cells. The relatively minute amounts bound by the cells are therefore not due to their impermeability, but instead reflect the inherently low reactivity of the cellular constituents with penicillin.
3. It is suggested that the relative non-toxicity of penicillin for the mammalian host, and for mammalian cells in tissue culture, may be related to this low order of reactivity with the antibiotic.