An assay for type III procollagen peptide (P-III-P) was applied to serum from 80 asbestos insulation workers in order to evaluate its relationship with pulmonary and pleural fibrosis. The serum peptide levels of 16 control workers were 7.8 +/- 2.0 ng/ml (mean +/- SD). An association between the serum values of P-III-P and the profusion of small irregular opacities on chest X-rays was noted. The levels in the group with moderate interstitial pulmonary fibrosis [profusion category 2(2/1, 2/2, and 2/3)] and with marked interstitial pulmonary fibrosis [profusion category 3(3/3)] were 9.9 +/- 2.3 ng/ml (n = 5) and thus tended to be higher than those of 16 control workers (p less than 0.1). On the other hand, there was not correlation between serum peptide levels and pleural lesions nor between the former and duration of asbestos exposure. From the results of the investigation [part I of which is reported elsewhere (Okazaki et al, 1986)] this assay may be useful for detecting individuals with a more marked tendency for progression of interstitial pulmonary fibrosis due to asbestos exposure in insulation work.