The subcommissural organ is an ependymal brain gland that secretes, into the ventricular cerebrospinal fluid, high molecular weight glycoproteins that form Reissner's fiber. Precursor and processed forms of secretion have been demonstrated by immunoblotting in the subcommissural organ of mammals and fish. In the chicken only a processed form has as yet been identified. In the present report, we have studied the subcommissural organ of 13-day-old chick embryos using (1) an antiserum against bovine Reissner's fiber, and (2) the lectins, concanavalin A and Limax flavus agglutinin. Paraffin sections of the subcommissural organ and blots of subcommissural organ extracts have been analyzed. The ependymal cells of sectioned subcommissural organ are strongly stained with the antiserum. Concanavalin A binds to materials in all cytoplasmatic regions, whereas Limax flavus agglutinin identifies materials confined to the apex of the ependymal cells. In the blots, a band of 540 kDa is immunostained. This band is positive for concanavalin A positive but negative for Limax flavus agglutinin and is thereby regarded as representing a precursor form of the secretion.