Summary Cryostat sections of bone marrow biopsies were stained immuno‐enzymatically for fibronectin using the alkaline phosphatase: anti‐alkaline phosphatase (APAAP) technique. In normal marrow fibronectin was present only in megakaryocytes and the walls of blood vessels. In contrast, myelofibrotic bone marrow possessed an extensive distribution of fibronectin which did not correspond to the distibution of reticulin. Infiltrated and hypercellular marrows showed an increase in fibronectin which appeared to be related to increased marrow vascularity. Again no correlation with the reticulin pattern was evident. In contrast to platelets, other circulating blood cells had no demonstrable fibronectin. This study suggests that fibronectin does not act as a mediator of haemopoiesis in vivo as has been suggested by the results of experiments in non‐human models.