Among marrow cells of rats only megkaryocytes stain appreciably for acetylcholinesterase. An investigation was therefore undertaken to determine whether this staining characteristic might be used to identify precursors of recognizable megakaryocytes. Marrow cells smaller than recognizable megakaryocytes were positive for the enzyme. The number of these cells increased after induction of severe thrombocytopenia by injection of antiplatelet antiserum. Subsequently, the number of recognizable megakaryocytes increased, suggesting their derivation from the small acetylcholinesterase-positive cells. These results suggest that acetylcholinesterase activity can be used as a marker for megakaryocytic cells that cannot be detected with conventional staining methods.