That there was a ‘Bacteriological Revolution’ in medicine in the late nineteenth-century, associated with the development of germ theories of disease, is widely assumed by historians; however, the notion has not been defined, discussed or defended. In this article a characterisation is offered in terms of four linked rapid and radical changes: a series of discoveries of the specific causal agents of infectious diseases and the introduction of Koch’s Postulates; a reductionist and contagionist turn in medical knowledge and practice; greater (...) authority for experimental laboratory methods in medicine; the introduction and success of immunological products. These features are then tested against developments in four important but previously neglected diseases: syphilis, leprosy, gonorrhoea and rabies. From these case-studies I conclude that the case for a Bacteriological Revolution in late nineteenth-century medicine in Britain remains unproven. I suggest that historians have read into the 1880s changes that occurred over a much longer period, and that while there were significant shifts in ideas and practices over the decade, the balance of continuities and changes was quite uneven across medicine. My argument is only for Britain; in other countries the rate and extent of change may have been different. (shrink)