Capillary zone electrophoresis was used to monitor the interaction between bilirubin and human serum albumin. Cord blood serum samples were injected directly into an uncoated fused-silica capillary (30 cm x 50 microns i.d.) and separation was accomplished within 4 min without extensive sample pretreatment. The most suitable running buffer to separate free bilirubin from albumin bound bilirubin was found to contain 1.0 mmol/L EDTA, 5% acetonitrile and 15 mmol/L phosphate with pH adjusted to 8.4. Approximately two bilirubin dianions could be bound per human serum albumin molecule in the cord blood serum. The binding constant was estimated to be 1.1 x 10(5) (L/mol) at 25 degrees C and pH 8.4. The peak area ratio of free bilirubin to total bilirubin can be used to determine the bilirubin binding capacity of cord blood serum for the concentration range of total bilirubin from 204 to 340 mumol/L using 1:5 diluted cord blood seras.