Prior reports describing the effects of lanthanum (La(3+)) on red blood cells (RBC) have focused on the effects of this lanthanide on cell fusion or on membrane characteristics (e.g., ion movement across membrane, membrane protein aggregation); the present study explores its rheological and biophysical effects. Normal human RBC were exposed to La(3+) levels up to 200 microM then tested for: (1) cellular deformability using a laser-based ektacytometer and an optical-based rheoscope; (2) membrane viscoelastic behavior via micropipettes; (3) surface charge via micro electrophoresis. La(3+) concentrations of 12.5 to 200 microM caused dose-dependent decreases of deformability that were greatest at low stresses: these rheological changes were completely reversible upon removing La(3+) from the media either by washing with La(3+)-free buffer or by suspending La(3+)-exposed cells in La(3+)-free media (i.e., viscous dextran solution). Both membrane shear elastic modulus and membrane surface viscosity were increased by 25-30% at 100 or 200 microM. As expected, La(3+) decreased RBC electrophoretic mobility (EPM), with EPM inversely but not linearly associated with deformability; changes of EPM were also completely reversible. These results thus indicate novel aspects of RBC cellular and membrane rheological behavior yet raise questions regarding specific mechanisms responsible for La(3+)-induced alterations.