Agglutination of human erythrocytes by the lectin concanavalin A is enhanced when the erythrocytes are pretreated with neuraminidase, which removes sialic acids, or with pronase, which degrades both the glycophorins and band 3 protein. In the present work transmission electron microscopy of the enzymatically pretreated erythrocytes shows a regular pattern of interruption of contact between interacting plasma membranes. The lengths characteristic of the pattern were 0.66 and 0.50 microns for pronase- and neuraminidase-pretreated cells, respectively. Agglutination of normal erythrocytes and of neuraminidase-pretreated erythrocytes can be fully reversed by exposure to the competitive inhibitor methyl alpha-D-mannopyranoside. Complete reversal of contact does not occur with pronase-pretreated cells. The comparatively greater tenacity of contact between cells that were treated with pronase before exposure to lectin argues for an involvement of nonspecific interactions in the agglutination process. The results are compared with previously published studies of spatially periodic contact patterns induced by a range of other polymers.