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Lipoprotein-X (LP-X) in cholestatic jaundice causes abnormal reaction in assays for low-density lipoprotein-cholesterol, but the effects on other test items are unknown. Here, we report an infant with biliary atresia showing abnormal reaction in total serum protein assay using the biuret method, and lipoprotein-X (LP-X) was then detected. In this 11-month-old female infant, jaundice was observed at 2 months old, and a diagnosis of biliary atresia was made. On biochemical tests at 12 months old, the total serum protein concentrations detected by the biuret method were very high, and the response curve and linearity of dilution were abnormal. LP-X was detected by agar electrophoresis. In addition and recovery experiments with normal serum fractionation of the patient's LP-X-rich lipoprotein fraction prepared by ultracentrifugation, normal γ-globulin fractionation showed an abnormal reaction by the biuret method. In infants with biliary atresia, we showed that the total serum protein assay by the biuret method was influenced by LP-X-rich lipoprotein, which may be caused by abnormal reaction of LP-X and γ-globulin. [Case Report].