A severe, pure deficiency of factor VII has been defined in haematological and biochemical terms in the Alderley Park colony of beagle dogs. By modification of the conventional factor VII assay to improve its specificity an apparent low activity of between 1 and 2.5% of the normal beagle has been found. When this plasma is used as a substrate for factor VII assays it compares well with human factor VII-deficient plasma in the measurement of both raised and depressed factor VII activity. It also appears to be a valuable reagent in the quality control of tissue thromboplastin extracts.