The distribution of TAL-1 protein, an important vascular promoter in mice, has been examined immunohistochemically in a range of human vascular lesions and normal tissues. Formalin-fixed, paraffin-embedded vascular lesions including granulation tissue, haemangiomas, Kaposi's sarcomas, spindle cell haemangioendotheliomas, and angiosarcomas, were examined using a monoclonal antibody to recombinant TAL-1. Endothelial cells in all lesions gave positive immunostaining of variable intensity. Granulation tissue and spindle cell areas of the vascular tumours gave the strongest staining (nuclear and cytoplasmic). The better-differentiated endothelial cells within the tumours and resident well-formed vessels were less positive and some cells were in fact negative. The malignant endothelial cells in angiosarcomas showed less intense positive staining than KS cells. This study has shown TAL-1 protein expression in a range of reactive, benign, and malignant vascular lesions. Protein expression appears to be stronger in the spindle cell areas, perhaps reflecting greater expression in less-differentiated endothelial cells.