A 52 year old woman had autoimmune hepatitis and an increased concentration of serum carbohydrate antigen 19-9 (CA19-9). The origin of the raised CA19-9 was studied using immunohistochemistry. Liver biopsy section showed chronic active hepatitis with large numbers of proliferated bile ductules. Immunohistochemical analysis revealed that the proliferated bile ductule cells were positive for proliferating cell nuclear antigen (PCNA) and for CA19-9. It is speculated that the raised serum CA19-9 concentration was derived from proliferated bile ductule cells and these cells, which are positive for PCNA, may be able to produce high concentrations of CA19-9. (J Clin Pathol 1998;51:167-169)