The Sephadex gel elution method for assay of the reserve bilirubin binding capacity (RBBC) of the serum of jaundiced newborns has been modified to permit its application on a routine basis in the clinical chemistry laboratory. The volume of sample required has been reduced to 0.2 ml. of serum or less, the time required to perform the assay decreased to 30 to 45 minutes, and the reliability of the results enhanced by unidormly higher absorbance readings. The RBBC as determined by this modified micromethod agrees within 1 mg./dl. with that obtained from the original method. The results are not affected by flow rates through the columns, which vary by as much as a factor of 5, but are interfered with by sample hemolysis, lipemia, and a high proportion of direct-reacting bilirubin. The RBBC assay is a useful tool in the management of neonatal hyperbilirubinemia, and the simplification of the method should permit its greater use on a routine basis.