The distribution of fibronectin and 140 Kd fibronectin-receptor was examined in the truncus arteriosus of the chick embryonic heart during aortico-pulmonary (AP) septation by immunohistochemistry. In 4-day-old chick embryos, immunoreactivity of both anti-fibronectin and anti-140 Kd fibronectin-receptor was seen in the primordia of the AP septum. In 5-day-old embryos, cells stained with anti-140 Kd fibronectin-receptor antibody were rarely found in the AP septum, though strong immunoreactivity of the anti-fibronectin antibody was observed in the AP septum. In 6-day-old embryos, at a late stage of septation, cells stained with anti-140 Kd fibronectin-receptor antibody were not found in the AP septum, and immunoreactivity of the anti-fibronectin antibody had decreased. In the cushion tissue, immunoreactivity of the anti-fibronectin antibody was seen, but that of the anti-140 Kd fibronectin-receptor antibody was recognizable only on a few cushion cells in the 4-day-old embryos. These findings suggest that fibronectin is involved in the formation of the AP septum primordia, that fibronectin does not promote the development of the AP septum, and that the migration mechanism of cushion cells is different from that of neural crest cells.