CD31 is a glycoprotein expressed on endothelial cells and in platelets. It is known to be involved in cell signaling and cell adhesion. JC/70A reacts with endothelial cells in normal tissues and in benign and malignant proliferations.1 In cryostat sections and blood smears the antibody also stains megakaryocytes, platelets and occasionally plasma cells.1 It reacts weakly with mantle zone B cells, peripheral T cells, and neutrophils. Antibody to CD31 is of value in the study of benign and malignant vascular tumors. Staining for CD31 has also been used to measure angiogenesis, which reportedly predicts tumor recurrence.