Blood serum concentration of procollagen type III peptides was assayed in 37 patients with chronic active hepatitis, 14 patients with persisting chronic hepatitis, and 11 normal subjects. Mean concentration of these peptides was significantly higher in patients with chronic active hepatitis than in those with persisting chronic hepatitis (25.6 +/- 11.5 ng/mL vs. 14.0 +/- 4.5 ng/mL; p < 0.001), and in individuals without lesions to the liver (25.6 +/- 11.5 ng/mL vs. 12.5 +/- 2.9 ng/mL; p < 0.001). Blood serum concentration of procollagen type III peptides may be helpful in the differential diagnosis of hepatitis.