It was not modesty but lack of conviction that kept Fleming silent on the therapeutic possibilities of penicillin for over ten years."" Thus concludes Macfarlane, Emeritus Professor of Clinical Pathology, Oxford, in this let's-get-the-record-straight biography. No easy task, because as his excellent scholarship and lucid exposition demonstrate, myth and mystery--some generated by Fleming himself--have created a giant often credited with the most important medical discovery to benefit mankind. Macfarlane builds his portrait slowly, carefully, from Fleming's modest beginnings in Scotland on to scholarships and prizes at medical school in London, and the almost casual acceptance of a job in Almroth Wright's Inoculation Department at St. Mary's Hospital, London. The leisurely pace, quiet competence, and steady achievement of our hero, though wearying at times, serve the biographer well in has demythologizing arguments. Two chance events occurred in Fleming's career. The first (often forgotten) was his discovery of the germicidal effects of lysozyme, an ingredient in tears, nasal secretions, saliva, and other body fluids. A drop of nasal mucus fell on a culture plate that happened to contain a rare bacterial species especially vulnerable to lysozyme action. The germs literally dissolved in moments. The second fortuitous happening was the effect of a particularly rare species of penicillium mold on colonies of staphylococci; and this event involved a series of individually improbable events--including the arrival of the rare mold spores, failure to incubate the bacterial culture, the weather in August of that year, and so on. . . not to mention Fleming's observation of the clear areas around the mold just when he was about to discard the culture. Fleming immediately set to work investigating the properties of the mold; he lost interest, however, when it appeared that the mold ""juice"" was slow-acting--and disappeared from the bloodstream too rapidly to do any good as a systemic antibacterial agent. He did pursue topical use of penicillin (a word he coined); but mostly he touted it as a useful reagent in the lab, a sort of selective weed-killer that could be used to isolate and culture pure strains of bacteria. Macfarlane gives all due credit to Howard Florey, Ernst Chain, and others at Oxford for discovering the clinical effectiveness of penicillin and for its later production in quantity. Honor for the discovery quickly came to Fleming, however--partly, Macfarlane suggests, because of the deliberate interventions of newspaper-mogul Lord Beaverbrook. In developing his theme, Macfarlane provides a brilliant sketch of Almroth Wright--a bear of a man who dominated bacteriology (and his disciples)--and provides model explanations in his reviews of the controversies surrounding microbes, antisepsis, immunology, and antibiotics back to the days of Pasteur, Lister, and Ehrlich. He takes issue with most Fleming biographies (except one by a Fleming colleague, Ronald Hare)--faulting Maurois, for example, for a false portrait of a brooding, unhappy Fleming surrounded by hostile rivals. Fleming was no dour Scot, Macfarlane avers, but a man who enjoyed a comfortable life, a happy marriage, wide circles of artist friends, lots of games and sports and just ""playing with microbes.