The tests on Gruinard Island carried out during the Second World War were but the first open-air trials to be conducted under the auspices of the British biological warfare project. Towards the end of the war, Paul Fildes had made approaches to his advisors and to the Navy to carry out weapon trials at sea as an extension of the land trials.1 Early strategic plans for the post-war biological warfare research programme included discussion of further trials and as to their status, the Superintendent of the MRD, Henderson, even entertained the notion that ‘no clear division’ could be drawn between offensive and defensive research, with the ‘possible exception of field trials’.2 I will argue in this chapter that the trials were significant, not simply for the time and resources devoted to them, but equally because they generated as many, if not more, problems than they solved for fundamental research. And, in the broader context, like the aspects of the programme described in the previous chapter, the series of trials held between 1949 and 1955 were a pivotal indicator of thinking not only about the nature of biological warfare but also its relative positions in Government, scientific and military policies.