  We discuss a locomotion of a three-sphere microswimmer in a viscoelastic
medium and propose a new type of active microrheology. We derive a relation
which connects average swimming velocity and frequency-dependent viscosity of
the surrounding medium. In this relation, the viscous contribution can exist
only when the time-reversal symmetry is broken, whereas the elastic
contribution is present only when the structural symmetry of the swimmer is
broken. The Purcell's scallop theorem breaks down for a three-sphere swimmer in
a viscoelastic medium.
