DURING the past few years, the measurement of the concentration of bilirubin in the serum has become very important in the pediatric field because of its value in the management of erythroblastosis fetalis and other forms of jaundice during early infancy. In previous communications, a micro modification of the standard method of Malloy and Evelyn was presented for the determination of the concentration of bilirubin in the serum of capillary blood. Although this method has proved to be very satisfactory, some have found the long waiting period for the development of the azobilirubin color a real objection. The present method overcomes this drawback by employing a dilute solution of caffein sodium benzoate as a catalyst, as first suggested by Jendrassak and Grof, and an increased strength of the diazo reagent as described by White and Duncan. It is well known that although the azobilirubin reaction continues to progress at a slower rate indefinitely, Malloy and Evelyn have shown that a 30-minute waiting period can be considered sufficient for the development of color when the ordinary diazo reagent is used. Using the 30-minute reading as the end point at which the reaction may be considered 100% completed, it can be seen (Fig. 1) that this same point is reached using caffein alone in 15 minutes, and using both caffein and a strengthened diazo reagent at 3 minutes. In the latter instance, the reaction does not change appreciably during the ensuing 26 minutes and may for all practical purposes be considered as complete at 4 minutes. [See the FIG. I. in Source pdf]