The concentration of the aminoterminal propeptide of type III procollagen (PIIINP) in serum is a measure of the activity of the metabolism of type III collagen, which is a major constituent of soft connective tissues and of the connective tissue stroma of solid tumours. In advanced ovarian carcinoma, serum PIIINP serves as a tumour-associated antigen, its changes reflecting and preceding changes in the clinical behaviour of the malignancy. Elevated values of serum PIIINP are also observed in endometrial and cervical malignancies, though less frequently than in ovarian tumours. Very high concentrations of PIIINP are found in ovarian carcinoma ascites, suggesting an ongoing fibro-proliferative reaction in the peritoneal cavity as a response to the tumour.