The authors assessed the test of platelet aggregation for the selection of a compatible donor. The test was compared with the hitherto used microlymphocytotoxic test. Consistent with data published abroad, the authors confirmed that the aggregation test improves the accuracy of selection of a suitable donor. The test can be performed in a shorter time than the microlymphocytotoxic test, this being important in particular in case of acute haemorrhage.