The haemostatic effect of 51Cr-labelled platelets was studied in 5 rabbits made thrombocytopenic (35,000/micronl blood) by whole body ionizing irradiation. Bleeding times were recorded after standardized cuts on the inner side of the rabbit's ear, a method with an acceptable reproducibility. The animals were then each transfused with concentrates of labelled platelets from 2 healthy donor rabbits. This increased the platelet counts to about 2 x 10(5)/micronl blood. Bleeding time values were markably prolonged before transfusion and became normalized when tested 1 and 4 h after transfusion. In 3 control experiments, where unlabelled platelet rich plasma was transfused to thrombocytopenic recipients, a similar shortening of the bleeding time was observed. It is concluded that 51Cr-labelled platelets retain haemostatic ability comparable to non-labelled platelets, when circulating in a recipient animal.