The present study was performed on s.c. adipose tissue of fetal pigs at 35 to 110 d of gestation to examine the distribution of TGF-β-positive cells, to localize TGF-β immuno-reactivity at the cellular level using electron microscopy (EM), and to determine the effect of TGF-β on primary cultures of pig adipose tissue cells. Tissues for EM were fixed and embedded in LR white resin. Sections then were incubated with a polyclonal antibody specific for TGF-β and TGF-β was located using 20 nm colloidal gold conjugated second antibody. Tissues were fixed and embedded in paraffin for localization of TGF-β at the light microscope (LM) level. Tissues were incubated with anti-TGF-β followed by localization using biotinylated second antibody. Using LM, only a few cells stained positively for TGF-β within developing Wood vessels at 35 d. By 50 d, more TGF-β-positive cells were associated with forming capillary networks. Between 70 d and 110 d, positively stained adipocytes usually were clustered around blood vessels. Cells surrounding hair follicles stained positive for TGF-β between 90 to 110 d. Electron microscopy revealed TGF-β labeling within fat cells. Fibroblasts and endothelial cells did not exhibit TGF-β immunoreactivity. The addition of TGF-β to primary cultures of s.c. adipose tissue cells from newborn pigs prevented lipid filling in fat cells. This effect was dose-dependent, with half-maximal inhibition occurring at 3 pM maximum inhibition occurred at 40 pM. These results indicate that TGF-β may regulate angiogenic activity and lipid filling in s.c. adipose tissue of fetal pigs. Although TGF-β was present in adipocytes and in cells associated with developing capillary networks, the physiological role of TGF-β during early adipose tissue development is not known.