During the last decades, livestock breeders attempted to develop strategies for decreasing fat content in meat. One of these strategies consists in immunization against adipocytes. Polyclonal antisera have been raised against fat cells or adipocyte plasma membranes of different species, and in vivo reduction of fat depots has been observed in some cases. Facing the problems of specificity and reproducibility inherent to polyclonal antisera, monoclonal antibodies could offer a promising alternative, which has been developed only in the porcine species. In our laboratory, a mouse monoclonal antibody of the IgG2b sub-class was raised against porcine adipocyte plasma membranes. This antibody did not cross-react in immunocytofluorescence with any tested cell-type or tissue other than adipocytes. Complement-mediated cytotoxicity was demonstrated in primary cultures of porcine stromal-vascular cells. When antibody and complement were added to already differentiated cultures, the treatment resulted in elimination of lipid-filled preadipocytes, whereas an early treatment of cultures prevented the appearance of these cells. Newborn piglets were injected at a time when most fat tissues are developing, except the intramuscular one maturation of which occurs later. The dose of 1 mg/kg of monoclonal antibody on d 2 and 5of life produced more than a 20% reduction of subcutaneous and leaf fat lipids at 35d of age, whereas the lipid content of muscle remained unaffected. In animals sacrificed at 50 and 100 kg, the early treatment with antibody led to a decrease of subcutaneous and visceral fat without affecting the growth performance of the animals. These results demonstrate that early systemic treatment of piglets with a specific anti-adipocyte antibody is able to reduce the fat mass, while keeping less affected the intramuscular fat depot.