Single, intravenously administered doses of cefoxitin and cefamandole did not penetrate into cerebrospinal fluid of normal humans. Multiple-dose administration with or without probenecid facilitated penetration of both antibiotics into the cerebrospinal fluid. Preliminary data showed that cefoxitin penetrated into cerebrospinal fluid of patients with inflamed meninges even when administered in a single dose without probenecid. However, the concentrations of cefoxitin in the cerebrospinal fluid of the individuals studied were not within the therapeutic range.