We have described previously a monoclonal antibody (SH2) that specifically recognizes undifferentiated mesenchymal progenitor cells isolated from adult human bone marrow. These cells, which we operationally refer to as mesenchymal stem cells, have the capacity to differentiate and form distinct mesenchymal tissues such as bone and cartilage when the isolated cells are placed in the appropriate in vivo or in vitro environment. We report here the partial biochemical characterization of the antigen recognized by the SH2 antibody. Metabolically radiolabelled adult marrow-derived mesenchymal stem cells in culture were extracted and immunoprecipitated with the SH2 antibody. The purified antigen migrated as a single band of 90 kDa after sodium dodecyl sulfate polyacrylamide gel electrophoresis was performed under reducing conditions. The SH2-immunoprecipitated protein exhibited a molecular weight band shift after removal of N-linked oligosaccharides. We investigated the expression of the SH2 antigen, along with the endothelial markers factor VIII-related antigen and Ulex europaeus I (UEA-I) lectin during specific developmental periods in human dermal embryogenesis and in the postnatal period through aged adults. Frozen sections of human embryonic, fetal, or postnatal skin ranging from 8 weeks estimated gestational age (EGA) through 84 years of age were immunostained or double immunolabelled with antibodies SH2, UEA-I, or factor VIII-related antigen followed by second antibodies with fluorescent markers. Positive cell surface reactivity with the SH2 antibody was seen in cells in the vascular plane in the earliest specimens (day 55 EGA) corresponding to the late cellular dermis period. During the period of the cellular to fibrous transition, in which the initiation of appendage development occurs, most SH2-reactive cells colocalized with vasculature markers UEA-I and factor VIII-related antigen, although there was a subset of cells recognized by SH2 antibody that did not colocalize with the endothelial markers. In contrast to the endothelial markers UEA-I and factor VIII-related antigen, in which the number of immunopositive cells became more prominent with age and maturation of the dermis, the frequency of cells that contained the SH2-reactive antigen diminished with age. The SH2 reactivity evident in embryonic, fetal, and early postnatal periods was not observed in human skin specimens taken from adults greater than 30 years old. These observations support the hypothesis that the SH2 antigen is a cell surface marker of developing microvasculature and may play a role in dermal embryogenesis and angiogenesis.