Plasmalemmal differentiation of the enucleating normoblast of rabbit and rat was studied by means of cytochemical methods and freeze-etching. Staining with colloidal iron revealed about identical amounts of iron particles bound to various areas of the normoblast membrane. Cationized ferritin and ruthenium red, likewise, failed in the demonstration of significant changes of the enucleating normoblast glycocalyx. Despite these findings the topo-optical staining with toluidine blue showed the plasmalemmal envelope of the protruding normoblast nucleus moderately birefringent, clearly discriminated from the intense anisotropic staining of the future reticulocyte membrane. The ferritin-labeled snail lectin anti AHP localized a great number of binding sites at the plasmalemmal envelope of the nucleus under extrusion. That is in sharp contrast with rather low lectin binding to the future reticulocyte membrane which amounts to about 30 to 50% of the nuclear envelope label. The findings provide evidence of unmasking of bindings sites of the normoblast membrane. Apparently, the effect is due to conformational changes of the cell membrane, rather than it could be attributed to degradation of glycoproteins. Moreover, enucleation kinetics may also be related to supramolecular changes of membrane structure albeit missing evidence for the rearrangement of membrane particles.