The ability of the influenza virus neuraminidase (NA) to cleave specific sialic acids was measured by cell electrophoresis. Most of the surface charge of human erythrocytes can be attributed to sialic acids. Therefore cleavage of sialic acids reduces the surface charge density which is measurable as a reduced cell electrophoretic mobility (EPM). For experiments specifically sialylated, erythrocytes were used. Their EPM was significantly decreased after incubation with virus strains possessing the corresponding NA specificity, even when the viral haemagglutinin (HA) was unable to bind to the erythrocyte's surface. Thus, the limited applicability of elution experiments, which requires virus binding, is overcome. An additional advantage of this procedure is that it is non-radioactive. In our model system the erythrocyte's surface resembles the natural situation of viral interaction with membrane-bound receptors.