Aggregation measurements were carried out with washed human erythrocytes in a NaCl-PBS [phosphate-buffered saline] solution under laminar shear conditions (D = 78.6 s-1) in a gap of a horizontally arranged, coaxial cylinder system. Aggregation was induced by LaCL3 at various concentrations and by concanavalin (Con) A (256 ng mm-3). After maximum degradation of sialic acid by neuraminidase (about 90% of all fixed negative charges of the surface coat), there was no spontaneous aggregation. Compared with untreated cells, the La3+-limit concentration still inducing an aggregation was reduced by about 1 order of magnitude. At a given La3+ or Con A concentration, an increased aggregation could be observed with increasing removal of sialic acids. The increase in effectiveness of these substances for inducing an aggregation was particularly striking at rates of sialic acid removal of > 50%. Results are discussed with special allowance for steric and structural force components of the surface coat.