We investigated the cells that express platelet-derived growth factor receptor α (PDGFRα) during mouse embryogenesis. PDGFRα expression has been identified by in situ hybridization or immunohistochemistry using polyclonal antibodies on tissue sections. Because no immunostaining study using whole-mount specimens has been published to date, we established a new monoclonal antibody (MAb), APA5, for this purpose. Our results differed in that APA5 stained only the paraxial mesoderm, whereas other investigators concluded that most if not all mesodermal cells expressed PDGFRα. Moreover, we did not find PDGFRα expression in embryonic erythrocytes, which have been previously suggested to express PDGFRα. On the basis of our present results, we wish to revise the proposed PDGFRα expression as follows. At the pregastrulation stage, PDGFRα is expressed only in primitive endoderm, particularly that in the ectoplacental cone. On gastrulation, it is expressed at high levels in the paraxial mesoderm. This expression continues after its differentiation into the somite. Along with the differentiation and migration of the sclerotome, PDGFRα⁺ cells begin to become distributed throughout the embryonal mesenchyme. During organogenesis, particularly intense staining is detected in regions of epithelial and mesenchymal interaction, such as the tooth bud and bronchi. In addition to mesodermal derivatives, the developing lens, apical ectodermal ridge, glial precursor, cardiac valves, and choroid plexus express PDGFRα. Our results with whole-mount immunostaining show that PDGFRα is abundantly expressed and may play important roles during embryogenesis.