  We present rheology experiments on dilute solutions of vesicles and red blood
cells (RBC). Varying the viscosity ratio $\lambda$ between internal and
external fluids, the microscopic dynamics of suspended objects can be
qualitatively changed from tank-treading ($tt$) to tumbling ($tb$). We find
that in the tt regime the viscosity $\eta$, decreases when $\lambda$ increases,
in contrast with droplet emulsions and elastic capsule theories which are
sometimes invoked to model RBC dynamics. At a critical $\lambda$ (close to the
tt-tb transition) $\eta$ exhibits a minimum before it increases in the tb
regime. This is consistent with a recent theory for vesicles. This points to
the nontrivial fact that the cytoskeleton in RBC does not alter the qualitative
evolution of $\eta$ and that, as far as rheology is concerned, vesicle models
might be a better description.
