There is limited information on the use of nanoparticles in medicine, and in particular, the effect of gold nanoparticles on human platelets. We carried out an experimental study at Chulalongkorn University, Bangkok, Thailand to demonstrate the direct effect of gold nanoparticles on platelets. A solution of gold nanoparticles was mixed with a blood sample and analysed. The results showed platelet aggregation. This finding could be useful for the development of new haemostatic drugs.