CD34, a single chain transmembrane glycoprotein, is selectively expressed on humanlymphoid and myeloid hematopoietic progenitor cells. Staining for CD34 has been used to measure angiogenesis, which reportedly predicts tumor recurrence. Antibody to CD34 also reacts with vascular endothelial cells in normal tissues and in benign and malignant proliferations. This antibody is of value in the study of benign and malignant vascular tumors as well as characterization of leukemias.