Endothelial cells are detected in a blood sample by enriching the endothelial cells from the blood sample followed by performing on the enriched endothelial cells an immunoassay capable of detecting antigens expressed by the endothelial cells. The immunoassay is capable of detecting antigen expressed from 300 endothelial cells per milliliter of blood. The method can be used for assaying mature circulating endothelial cells or circulating endothelial progenitor cells.
