Summary Collagen and acid glycosaminoglycans in the skin of progressive systemic sclerosis (PSS) were examined by polarization microscopy. Picrosirius Red and Toluidine Blue (pH 5.8) were used as stains. Digestion with chondroitinase ABC or streptomyces hyaluronidase were also employed. Under polarized light, the Picrosirius Red-stained collagen appeared green at any stage in PSS and orange in controls. Toluidine Blue-induced birefringence at stage I diminished in the presence of 0.2 M MgCl 2 and in stage II in the presence of 0.3 M MgCl 2. The collagen fibrils in PSS skin were significantly smaller in diameter than in controls. These results suggest that the change of polarization colours is due to the modulation of collagen thickness caused by an increased accumulation of acid glycosaminoglycans.