Different low molecular weight (LMW) heparins were tested on primary haemostasis in rats. Four preparations were studied; one was devoid of any effect on the bleeding time, while the other three prolonged the bleeding time to varying extents. As a consequence we studied the effect of these heparins on platelet aggregation. The fractions which prolonged the bleeding time, also inhibited the ex vivo and in vitro platelet aggregation, whereas the one devoid of any effect on the bleeding time did not affect platelet aggregation. Similar results were obtained using both platelet-rich plasma (PRP) and gel-filtered platelets. The in vitro response of platelets to aggregating agents may offer a parameter to detect the presence of 'bleeding factor(s)' in some LMW heparin preparations.