AbstractRecent studies suggest that the serum level of Type III procollagen (PC‐III) could be a valuable, noninvasive monitor of hepatic fibrogenesis. We have developed a sensitive and specific radioimmunoassay for PC‐III procollagen isolated and purified from fetal goat skin which shows high cross‐reaction for human PC‐III. In a double‐blind study, serum samples taken from 8 normal volunteers and 50 patients at the time of liver biopsy were assayed for PC‐III. Each biopsy was reviewed without knowledge of the radioimmunoassay results, and old or active fibrosis was graded on a 0 to 3 (none, minimal, moderate, severe) scale. There was a significant difference between serum concentrations of PC‐III in normals (mean ± S.D. = 72 ng per ml ± 8) as compared to patients with biopsy evidence of active fibrosis (mean ± S.D. = 153 ng per ml ± 12; p<0.001). The serum concentration of PC‐III showed a good correlation (r = 0.58; p<0.001) with the histological grade of active lobular fibrosis. Measurement of PC‐III is a noninvasive test for detection of active