THE Linacre Lecture, 1946, delivered by Sir A Alexander Fleming, on “Chemotherapy, Yesterday, Today and Tomorrow” (Cambridge University Press, 1946. 2s.), has now been published. Opening his lecture with the warning that the title chosen was too ambitious and would require a series of lectures, Sir Alexander limited his remarks to chemotherapy of which he has had first-hand knowledge. He refuses to confine the meaning of the term 'chemotherapy' to the administration of chemical substances by way of the blood; he extends it to include any form of treatment which enables a chemical substance to exert directly an injurious effect upon bacteria. This definition includes antiseptic treatment, two cardinal principles of which are the avoidance of chemical substances which are too toxic to the host or to the tissues, and the effective diffusion of the therapeutic agent into the infected tissues. Discussing the antiseptic methods of Lister and the practice of asepsis which followed them, Sir Alexander demonstrates, by reference to experience gained during the First World War, how important it is that diffusion of the local chemotherapeutic agent into the infected area should occur and, by reference to the mode of action of Dakin's solution, how important is the drainage of the infected lesion by the exudation of fluid from it. Important also is the action of local chemotherapeutic agents upon the phagocytes which attack the bacteria. Sir Alexander's own work on this problem is well known, and he here shows how essential it is to consider the speed of action of the chemical substance on the bacteria and on the phagocytes respectively. If the phagocytes are killed by it before it kills or appreciably inhibits the growth of the bacteria, the tissues are left without one of their main means of defence.