Fetal antigen 2 (FA2) in human fetal osteoblasts, cultured osteoblasts and osteogenic osteosarcoma cells. Immunohistochemical staining techniques used on an 11-week-old fetus showed that fetal antigen 2 (FA2) was present intracellularly in endochondral and perichondral osteoblasts, and the immunoreaction was extended into the adjacent bone matrix. Osteoclasts and chondroblasts were found to be FA2 negative. A granular perinuclear intracytoplasmic FA2 immunoreaction was found in cultured osteoblasts and osteogenic osteosarcoma cells, and immunoelectron-microscopical examination revealed a granular immunoreaction product in the rough endoplasmic reticulum. These findings indicate that FA2 is synthesized by osteoblasts and osteogenic osteosarcoma cells. A reaction of immunological identity was found between FA2 purified from second trimester amniotic fluid and serum-free supernatants of cultured osteogenic osteosarcoma cells. This shows that an antigen recognized by the anti FA2 antibody is secreted by these malignant cells. Thus, FA2 may represent a marker for altered bone metabolism, and have a potential in the classification of osteogenic osteosarcoma/chondrosarcoma.