The marginal band (MB) of nucleated erythrocytes is composed of a bundle of microtubules that encircles the cell immediately beneath the plasma membrane. When cells are lysed in buffer containing Triton X-100 the MB microtubules remain intact, and the nucleus remains suspended at the cell center by a filamentous network called the trans-MB material that connects the nucleus to the peripheral MB. When these lysed cells are prepared for indirect immunofluorescence by use of an antibody to chick brain microtubule-associated protein 2 (MAP 2), intense staining of the MB results; no staining is evident in the areas occupied by the nucleus or the trans-MB material. Controls demonstrate that the staining is specific, because no staining occurs with fluorescent goat antirabbit serum alone or when nonimmune serum is used as the first antibody. Furthermore, the fluorescence of the MB is not affected by pretreatment of the immune serum with purified tubulin, but staining is prevented by pretreatment of the immune serum with purified MAP 2. To determine which protein component of the MB was responsible for the positive immunofluorescence results, 125I-protein A staining was used after the protein components of the isolated cytoskeleton had been resolved by SDS-polyacrylamide gels. Controls showed that the antiserum could react on SDS gels with MAP 2 from purified chick brain microtubules. The results with the cytoskeletal proteins demonstrated that the antiserum reacted only with a high molecular weight protein having a molecular weight similar, but not identical, to that of chick brain MAP 2. Thus, it is concluded that a protein with antigenic characteristics similar to those of chick brain MAP 2 is a component of the MB. The results are discussed in terms of the possible function of MAP 2 in the MB.