In the years around 1900 one of the most significant practical consequences of new styles of bacteriological thought and practice was the development of preventive vaccines and therapeutic sera. Historical scholarship has highlighted how approaches rooted in the laboratory methods of Robert Koch, Louis Pasteur and their collaborators were transformed in local contexts and applied in diverse ways to enable more effective disease identification, prevention and treatment. Amongst these, the anti-anthrax serum developed by the Italian physician Achille Sclavo (1861–1930) has (...) received little to no attention from historians. This article positions Sclavo's serum as a neglected but significant presence in British microbiology, which achieved widespread uptake amidst a wave of optimism, despite prolonged uncertainty about its mechanism of action and dosage. After being introduced to Britain in 1904 by the enterprising first medical inspector of factories Thomas Morison Legge, within a matter of months the serum became regarded by medical practitioners as an effective treatment of cutaneous anthrax, though access to ‘fresh’ serum and the necessary speedy diagnosis remained problematic. Like the disease anthrax itself, discussion of ‘Sclavo's serum’ was out of all proportion to the relatively low number of cases, reflecting a deep-seated fascination with the wider possibilities afforded by effective serum therapy. (shrink)