THE EFFECT of penicillin and streptomycin on bacterial infections of the subarachnoid space has in general been beneficial, and the use of these agents by intrathecal administration has become widespread. A note of warning has been sounded by a number of authors,1 who have described clinical examples of unfavorable results attributable to the direct toxic effect of penicillin or streptomycin on the cerebral tissues or their meningeal coverings. In experimental investigations, this direct effect of chemotherapeutic agents on the cerebral tissues has been studied carefully,2 and the effect of penicillin on experimentally produced meningeal inflammations has also been evaluated.3 There does not appear to have been any comment on the effect of penicillin and the other chemotherapeutic agents in altering or modifying the whole pathologic sequence of the development and resolution of human meningitides. With this end in view, a study was made of 16 cases of