While the transcutaneous bilirubinometer (TcB) was originally developed for use as a jaundice meter, we have shown that it can also be used as a bedside instrument for direct measurement of serum bilirubin. An analysis of 130 samples indicates that total serum bilirubin measurement using the TcB is simple and accurate. A comparison with the AO bilirubinometer showed a highly significant coefficient or correlation (r = 0.99) for total serum bilirubin concentration. The addition of hemolysates caused no effect on TcB reading obtained by the TcB instrument, even at levels above those usually encountered in sera of newborn infants. The AO cuvette is very suitable for use in this measurement, since this method requires less than 15 microliters of serum. One of the disadvantages of this method may be that it yields somewhat lower (5%) than actual serum bilirubin concentrations in the range above 20 mg/dl, and somewhat higher (20%) than actual serum bilirubin concentrations in the range below 10 mg/dl. Our study demonstrated the TcB instrument to be a useful device for bedside determination of serum bilirubin in the screening of neonatal hyperbilirubinemia. However, though useful as a screening test, it does not provide such an accurate determination of serum bilirubin concentration as the AO bilirubinometer.