Thirteen breast-fed one-month-old infants with prolonged jaundice not due to known causes were included in this study. All infants were investigated at one and twelve months of age. Serum concentrations of total (TB) and conjugated bilirubin (CB), aspartate (ASAT) and alanine aminotransferase (ALAT), alkaline phosphatase (AP), alpha-1-antitrypsin (alpha-1-AT), alpha-1-fetoprotein (AFP) and the two primary bile acids; cholic (CA) and chenodeoxycholic acid (CDCA) were determined at both ages. The Pi-phenotype of alpha-1-AT was determined at the age of twelve months. The serum concentrations of TB, CB, AP and AFP were elevated at the age of one month but were normal at the age of twelve months. No changes in the serum concentrations of ASAT or ALAT were observed between one and twelve months of age, and the values were within the reference ranges. The serum concentrations of alpha-1-AT were within the reference range at both ages. Two infants were heterozygous for MZ, and they had normal serum alpha-1-AT concentrations. The serum concentrations of CA and CDCA were elevated at the age of one month and were still significantly elevated at the age of twelve months indicating that the infants had slight cholestasis at the age of one month, and that the cholestasis had largely subsided by the end of the first year of life.