A glycopeptide, isolated from bovine cerebral cortex cells and added to cells in only nanogram/ml levels, has been shown to inhibit both cell protein synthesis and cell division. A monoclonal antibody was used to show that the inhibitory component originated from the cell surface. Incubation of the M1 IgG monoclonal antibody with partially purified bovine glycopeptide preparations and Staphyloccocus protein A removed the inhibitor from solution. Intact mouse cerebral cortex cells were found to have a similar epitope on their surfaces. In contrast, normal rat kidney cells (NRK) did not react with the monoclonal antibody. An analysis of mouse cerebral cortex membrane preparations, incubated with the monoclonal antibody, confirmed that the primary source of the antigenic determinant was the plasma membrane.