We have evaluated the usefulness of the PFA-100 system (collagen/ADP and collagen/epinephrine cartridges) to assess the in vitro effects of a few platelet function inhibitors: Aspisol (60 microg/ml), 4-[4-[4-(aminoiminomethyl]-1-piperazinyl]-1-piperidineactetic acid, hydrochloride trihydrate (GR144053F, fibrinogen receptor antagonist, 100 nM), adenosine-3',5'-diphosphate (A3P5P, P2Y1 ADP receptor antagonist, 500 microM) and Bis[(adenosine-5'-O-phosphorodithioyl)methylene]-phosphinic acid (APTMPA, P2Y12 ADP receptor antagonist, 500 microM) on platelet function, as compared with the other commonly used diagnostic technique, a whole blood electrical aggregometry (20 microM ADP or 0.5 mM arachidonic acid). The in vitro studies were carried out on a group of 38 subjects. Whereas all the examined platelet antagonists and inhibitors almost completely blocked the 20 microM ADP- or 0.5 mM arachidonic acid-induced (in the case of acetylsalicylic acid) whole blood aggregation, only two inhibitors (Aspisol and GR144053F) remained effective in a significant prolongation of the PFA-100 occlusion time. Otherwise, using the PFA-100 system we were not able to detect the inhibitory actions of ADP receptor antagonists- P2Y1 and P2Y12. Our findings point to a limited usefulness of the PFA-100 system for the monitoring of the effectiveness of ADP receptor antagonists. The outcomes of this study show that platelet aggregometry in whole blood is characterised by the highest sensitivity in the monitoring of the investigated blood platelet inhibitors.