Drug resistance or "fastness," that is the acquisition by micro-organisms of tolerance for chemotherapeutic agents, has long engaged the attention of bacteriologists and parasitologists1and has recently become a matter of concern among clinicians and epidemiologists. Resistant strains of micro-organisms usually susceptible to the sulfonamides have been recognized for some time, and penicillin resistant strains are now beginning to appear in clinic and laboratory. The apprehension which might have been caused by the latter is currently being allayed by the introduction of streptomycin. Experiments which we2have recently completed have demonstrated that meningococci and gonococci acquire resistance to penicillin during cultivation on mediums containing penicillin, provided its concentration is increased carefully at each transfer and kept below the level which inhibits active multiplication of the micro-organisms. By observing these precautions, the penicillin tolerance of a strain of gonococcus has been raised to a degree which allows it to