Platelets are involved in anti-parasitic mechanisms. This question is poorly recognized. Platelets may exhibit this function only after activation. Therefore we conducted this study measuring some morphological parameters of these cells which reflect activation of platelets. The experiments were done on 30 patients infected with Giardia intestinalis. Diagnosis was supported on finding trophozoits in bile, cysts and GSA 65-protein in feces. We have found some decrease of blood platelets count and normalization after anti-parasitic treatment. In the line with this observation is platelet crit. A slight change was noted in mean platelet volume before and after treatment if we compare to controls. The same observation concern platelet anisocytosis index. Nearly two-fold decrease in the count of large platelets was observed before and after treatment 3.77% and 2.07%. Our general conclusion is, the parasite is able to activate the platelets and anti-parasitic treatment normalize these changes.