In vitro human platelet interactions with surfaces of type I collagen, chondroitin-6-sulphate (CH-6-S), chondroitin-4-sulphate (CH-4-S), a CH-6-S/collagen layer, and a collagen-CH-6-S complex were investigated. Polystyrene and silanized glass served as controls. Platelet counts, platelet factor 4 released, and platelet aggregating ability for the different surfaces were compared with controls. Platelet count and platelet factor 4 release data showed that there were no differences between surfaces of CH-6-S, CH-4-S and the controls. However, significant differences in platelet counts and platelet factor 4 released were found when collagen, the CH-6-S/collagen layer, and the collagen-CH-6-S complex were compared with controls. The pure type I collagen surface had the greatest influence on platelet activation. The collagen-CH-6-S complex had a greater effect that the CH-6-S/collagen layer on platelet activation. It appears that chondroitin-6-sulphate can modify the platelet activity of type I collagen.