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Formalin-fixed, paraffin-embedded material from 15 haemangioendotheliosarcomas and eight malignant haemangiopericytomas was stained with an immunohistochemical technique for the presence of factor VIII-related antigen (F VIII-RAg), actin, laminin and for reactivity with the lectin Ulex europaeus I (UEA-I). Haemangioendotheliosarcomas stained with both F VIII-RAg and UEA-1; however, UEA-I was found to be the more sensitive of the two, reacting also with the poorly differentiated tumours. Haemangiopericytomas reacted negatively with UEA-I; surprisingly, some of these tumours exhibited a weak positivity in staining for F VIII-RAg, possibly supporting the theory about intermediate forms between haemangioendotheliosarcomas and haemangiopericytomas. Laminin was found in most of the haemangioendotheliosarcomas and was useful in illustrating their vascular growth pattern. Haemangiopericytomas also reacted positively for laminin, but the intensity of staining was less pronounced. Positive staining for actin was demonstrated in both tumour types.