Fat and bone metabolism are two linked processes regulated by several hormonal factors. FA1 (fetal antigen 1) is the soluble form of dlk1 (delta like 1), which is a member of the Notch-Delta family. We have previously identified FA1 as a negative regulator of bone marrow mesenchymal stem cell differentiation. Here, we studied the effects of circulating FA1 on fat and bone mass in vivo by generating mice expressing high serum levels of FA1 (FA1-mice) using the hydrodynamic-based gene transfer procedure (HGTP). We found that increased serum FA1 levels led to a significant reduction in total body weight, fat mass and bone mass in a dose-dependent manner. Reduced bone mass in FA1-mice was associated with the inhibition of mineral apposition rate and bone formation rates by 58% and 72% respectively. Since FA1 is co-localized with growth hormone (GH) in the pituitary gland, we explored the possible modulation of serum FA1 by GH. Serum levels of insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBPs) did not change in FA1-mice, while increasing serum GH in normal mice using HGTP, dramatically reduced serum FA1 levels by 60%. On the other hand, serum FA1 was increased 450% in hypophysectomized mice and this high level was reduced by 40% during GH treatment. In conclusion, our data identify the FA1 as a novel endocrine factor regulating bone mass and fat mass in vivo and its serum levels are regulated by GH. FA1 thus, provides a novel class of developmental molecules that regulate physiological functions of the post-natal organisms.