The aim of this study was to investigate the effects of various gelatin and hydroxyethyl starch solutions on platelet reactivity. Citrated whole blood was obtained from 20 healthy volunteers. Expression of glycoprotein (GP) IIb–IIIa and p-selectin were determined using whole blood flow cytometry on both resting and agonist-activated platelets before and after in vitro haemodilution (20% and 40%) using oxypolygelatin, modified gelatin, urea-linked gelatin, hydroxyethyl starch (HES) 130 (mean molecular weight in kDa), HES 200, HES 450 and HES 550. High degrees of haemodilution using oxypolygelatin had no significant effect, similar to HES 130, whereas modified gelatin inhibited GP IIb–IIIa expression, similar to HES 200 and HES 450. Urea-linked gelatin significantly increased the expression of GP IIb–IIIa, similar to HES 550. p-Selectin expression remained unchanged in all samples. The present in vitro study indicates that chemical characteristics of colloidal solutions modulate their influence on platelet reactivity.