We recently identified circulating osteoblastic cells using antibodies to osteocalcin (OCN) or alkaline phosphatase (AP). We now provide a more detailed characterization of these cells. Specifically, we demonstrate that 46% of OCN positive (OCN(pos)) cells express AP, and 37% also express the hematopoietic/endothelial marker CD34. Using two different anti-OCN antibodies and forward/side light scatter characteristics by flow cytometry, we find that OCN(pos) cells consist of two distinct populations: one population exhibits low forward/side scatter, consistent with a small cell phenotype with low granularity, and a second population has higher forward/side scatter (larger and more granular cell). The smaller, low granularity population also co-expresses CD34, whereas the larger, more granular cells are CD34 negative. Using samples from 26 male subjects aged 28 to 68 years, we demonstrate that the concentration of circulating OCN(pos) cells increases as a function of age (R=0.59, P=0.002). By contrast, CD34(pos) cells tend to decrease with age (R=-0.31, P=0.18); as a consequence, the ratio of OCN(pos):CD34(pos) cells also increase significantly with age (R=0.54, P=0.022). These findings suggest significant overlap between circulating cells expressing OCN and those expressing the hematopoietic/endothelial marker CD34. Further studies are needed to define the precise role of circulating OCN(pos) cells not only in bone remodeling but rather also potentially in the response to vascular injury.