A microdetermination of neonatal serum bilirubin concentration by AutoAnalyzer consumes less than 100μliters of sample for two tests, requires less than 15 minutes to equilibrate the instrument and less than six minutes between time of sampling and time of read‐out. It provides especially accurate data between 10 and 25 mg bilirubin/100 ml and useful data between zero and 40 mg bilirubin/100 ml. The instrument is sufficiently simple to permit its operation on a 24‐hour basis by blood bank technologists not previously experienced in clinical chemi