Chick serum contains a factor that stimulates adipogenesis in Meckel's chondrocytes in vitro. The present study examined whether chick serum has a capacity for adipogenic induction in vivo, by transplanting serum clots (created by drying chick serum for up to 4 weeks) into mouse spleens. Specimens were harvested for histological analyses, which included light and electron microscopy and immunohistochemistry. The transplanted serum clots induced the appearance of lipid droplet-containing cells in splenic cords and sinus. Almost all the lipid droplet-containing cells were positive for sudan staining and consisted of multilocular lipid vacuoles. Immunostaining showed that the adipocytes induced by transplantation of the serum clots initially appeared as peroxisome proliferator-activated receptor-gamma (PPARgamma)-positive cells and developed into leptin and alpha-glycerophosphate dehydrogenase (GPDH)-producing cells, in addition to type III collagen synthesis. Furthermore, double immunofluorescence staining revealed that the immunoreactivity for GPDH was detected not only in stromal cells but also in macrophages. It was thus confirmed that stromal cells and macrophages in the spleen contain lipid droplets as seen in intact white adipose cells. The present results suggest that chick serum contains factors for adipocyte induction not only in vitro but also in vivo, and that the adipogenic potential does not depend on the supplements used during the cell culture.