The serum concentration of the N-terminal peptide of type III procollagen (PIIINP) was determined in 32 patients with myelomatosis (MM). Four subjects were studied at the time of diagnosis and the remaining patients at variable intervals from diagnosis. Serum concentration of beta-2-microglobulin (B2m) was measured in 31 patients. Serial measurements of both substances were performed in 20 patients. Serum PIIINP was increased in MM as compared with healthy control subjects (P < 0.001). Patients with active disease had significantly higher propeptide values (median 7.4; range 3.8-11.2) as compared to those with stable disease (median 4.3; range 2.2-9.6) (P < 0.009). A highly significant correlation existed between circulating PIIINP and B2m (P < 0.001). It is concluded that MM elicits a stromal reaction as reflected by parallel increases in serum PIIINP and serum B2m. In subsets of patients, e.g. those with non-secretory myeloma and in patients with smouldering disease, serum PIIINP may even be superior to B2m as an indicator of disease activity.