Determination of blood group isoantigens A, B and H, was performed in benign and malignant vascular tumours of the skin and subcutaneous tissue, using the immunoperoxidase technique. No differences were noted between benign haemagioendotheliomas from children or adults: neither tumour showed the presence of antigens in intercapillary cells. Reactive conditions such as angiolymphoid hyperplasia with eosinophilia showed an intense positive reaction to blood group substances of the endothelial proliferative cells. In malignant tumours no relationship between tumour differentiation and loss of blood group isoantigens was seen. Cases of Kaposi's sarcoma did not show antigens in spindle cells or capillaries but in medium sized vessels variable preservation or loss of blood group isoantigens was found.