The transcutaneous bilirubinometer was evaluated in 43 white infants, eight black infants, and in nine white infants treated with phototherapy. The reproducibility of the instruments was determined after trials consisting of both five and 100 repetitions. Among the infants not being treated with phototherapy, TcB index and serum bilirubin concentration correlated at 0.90 level in both white and black infants. Phototherapy reduced the accuracy of the TcB and, at the present time, the use of this index in infants under light therapy cannot be recommended. Otherwise the TcB is a valuable tool in screening healthy term infants for hyperbilirubinemia.