The adhesion of human erythrocytes to glass in the presence of the polycation, polylysine, has been examined. The study provides experimental support for earlier predictions (D.S. Dimitrov, Colloid Polym. Sci., 260 (1982) 1137; D. Gallez, Colloids Surfaces B: Biointerfaces, 2 (1994) 273) that the aqueous layer between a membrane and a solid substratum can become unstable, giving a spatially periodic distribution of contact regions. The morphology and peripheral contacts of the adhered cells were examined by light and scanning electron microscopy (SEM). Cells were exposed in suspension to 1.0 or 5.0 pg ml-' of 193 or 205 kD polylysine and then allowed to settle on glass. The adhered cells generally lost their normal biconcave shape and spread over the glass surface. Localised regions of adhesion were identified at the edges of the spread cells. Adhered cells were fixed with glutaraldehyde and their undersides were exposed for SEM examination. The contact seam was interrupted by a pattern of concavities on the erythrocyte underside. The contact seam patterns and their typical lateral separation, about 1 pm, were similar to those previously observed when erythrocytes interacted with each other in suspension in the presence of polylysine. The lateral separation distance decreased with increasing polycation concentration. The SEM observations of the present work are compared with light microscopy observations of local contact formation in plasma-membrane/glass interactions.