The long-term success rate of coronary angioplasty is only 50 to 70% due to restenosis. The pathophysiologic mechanism of this event is mainly mediated by smooth muscle cell proliferation. To some extent, it can be inhibited by heparin. The introduction of LMWHs allows for prophylaxis against restenosis on an outpatient basis, but no data are available on the long-term usage of LMWH. To investigate possible side effects, nonhuman primates received daily injections of LMWH (Mono-Embolex) over a 12-week period and the hematologic profile was assessed. There were no significant alterations of the red blood cell system. The overall WBC profile did not change, although a significant increase of granulocytes with a corresponding decrease of lymphocytes and monocytes occurred. Data from a control group suggest that these alterations are caused by chronic stress and are not drug related. Platelet count as well as platelet function tests were unaffected by this treatment. In conclusion, the long-term administration of Mono-Embolex as projected for the use in the prophylaxis of restenosis following angioplasty appears to be safe regarding hematologic parameters.