Platelet aggregation in whole blood was investigated in patients with Systemic sclerosis and age- and sex-matched controls. Dose-response curves for collagen, adrenaline and ADP-induced fall in platelet count were constructed. Aggregation to collagen at all concentrations was significantly greater (p less than 0.01) in the patients with systemic sclerosis than the normal controls, with a four-fold reduction in the ED50 for SS (0.044 +/- 0.03 mcg/1) compared with controls (0.12 +/- 0.008 mcg/1). No significant difference was observed in the response to the other aggregating agents, thus suggesting that in this disease a platelet abnormality exists which is specific for collagen. Increased platelet responsiveness to collagen and hence increased release of platelet-derived growth factors may provide a lin between endothelial damage and the connective tissue fibrosis of systemic sclerosis.