Transforming growth factor-beta 1 was localized by means of immunohistochemical reaction in liver biopsy specimens taken from patients having different chronic liver diseases with extending fibrosis. Two polyclonal antibodies that were produced in rabbits were directed against the amino terminal of transforming growth factor-beta 1. Staining by anti-CC(1-30) was primarily extracellular and located in the portal and periportal fibrotic areas of all seven cases with chronic active hepatitis. No staining was noted in the four chronic persistent cases studied. A strong reaction was seen with the antibody in nine of the ten cirrhotic samples, whereas it was negative in one inactive cirrhosis case and in all five cases with normal liver histological findings. No positive staining could be detected by the anti-LC(1-30) in any of the liver tissues. Detection of transforming growth factor-beta 1 in active liver diseases at the site of fibrosis suggests that transforming growth factor-beta 1 might have a role in the process and progression of fibrosis during the development of the disease.