London County Council's pathological laboratory in the LCC asylum at Claybury, Essex, was established in 1895 to study the pathology of mental illness. Historians of psychiatry have understood the Claybury laboratory as a predecessor of the Maudsley Hospital in London: not only was this laboratory closed when the Maudsley was opened in 1916, but its director, Frederick Walker Mott, a champion of the 'German' model in psychiatry, was instrumental in the establishment of this institution. Yet, as I argue in this essay, for all the continuities with the Maudsley, the Claybury laboratory should not be seen solely as its predecessor - or as a British answer to continental laboratories such as Theodor Meynert's in Vienna. Rather, as I show using the examples of general paralysis of the insane and 'asylum colitis', the Claybury laboratory is best understood as an attempt to prevent mental illness using a microbiological model.