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(Abstracted from the British Medical Journal, ii, 6th October, 1945, p. 464) Though its specificity is rather like that of the sulphonamides, its mode of action is different. For example, it is not antagonized by p-aminobenzoic acid, peptones or pus; the sulphonamides are. Further, penicillin can prevent the growth of sulphonamideresistant organisms, and the sulphonamides can inhibit the growth of penicillin-fast strains; and whereas the sulphonamides act principally by slowing down the rate of growth, penicillin can cause rapid death and lysis of susceptible micro-organisms. It is generally held that penicillin has three phases to its action. It may be bacteriostatic, bactericidal, or lytic, according to conditions. Veiy small amounts, down to one-tenth of the bacteriostatic dose, may cause pronounced morphological changes. Cocci swell to a