RADIOACTIVITY in blood glucose following injection of 14C-labelled precursor has been determined during numerous metabolic investigations. In most cases, either a comparatively large volume of sample1 or a considerable amount of carrier glucose2 has been used to prepare a sufficient quantity of glucosazone. Both these methods have disadvantages. Obtaining a large blood sample is nearly impossible in small animals and also has serious limitations on repeated sampling in large animals, while the addition of carrier glucose is limited to blood glucose which has a relatively high specific activity. Some investigators have counted glucosazone for radioassay at infinite thickness3 while others have converted it to CO2 and counted it as BaCO3 at infinite thickness1.