The aim of this study was to compare serum N-terminal peptide of type III procollagen to aminotransferases and gamma-globulins as a marker for histological activity in patients with chronic hepatitis and to assess the role of type I collagen, a new serum marker, as a marker of fibrosis in these patients. Sixty patients with biopsy-proven chronic hepatitis were included in this study. Liver disease was virus B-related in 29, autoimmune in five, drug-induced in five, and of unknown etiology in 21. Each biopsy was independently assessed by two liver pathologists. Two histological scores, a score of activity and a score of fibrosis, were established. Serum N-terminal peptide of type III procollagen and type I collagen were assayed by liquid phase RIA. Significant correlations were noted between serum N-terminal peptide of type III procollagen and scores of activity (r = 0.70, p less than 10(-4)) and fibrosis (r = 0.45, p = 0.0005), and between serum type I collagen and scores of activity (r = 0.46, p = 0.0004) and fibrosis (r = 0.67, p less than 10(-4)). When the correlation between scores of activity and fibrosis (r = 0.52, p = 10(-4)) was considered by partial correlation, serum N-terminal peptide of type III procollagen was correlated with the score of activity (r = 0.63, p less than 10(-3)) but not with the score of fibrosis, and serum type I collagen was correlated with the score of fibrosis (r = 0.58, p less than 10(-3)), but not with the score of activity.(ABSTRACT TRUNCATED AT 250 WORDS)