The history of the emergence of clinical pathology laboratories after 1890 has been written around the growth of services provided by public health agencies and teaching hospitals. In this paper I discuss a neglected aspect of development for such services – those provided by private companies – and in particular the work of one London-based enterprise – the Clinical Research Association Limited (CRA). The current historiography of clinical pathology laboratories has two characteristics that this paper challenges: first, that the provision of such laboratories was supply-led, in other words, that services were developed by enthusiasts for laboratory medicine ahead of demand by clinicians. And second, that these enthusiasts stressed that the new laboratory methods were superior to traditional clinical methods of diagnosis. Instead, I will argue that the establishment and growth of the CRA was due to the entrepreneurial exploitation of a clear, though quite specific, demand from doctors and their patients; and second, that the promoters of the CRA went out of their way to avoid challenging clinical authority, seeking an accommodation where laboratory data was to be just another factor in a diagnosis.