  The effective electromagnetic current density for a two-nucleon system that
is described by the Blankenbecler-Sugar equation is derived. In addition to the
single nucleon currents there are exchange currents of two different origins.
The first is the exchange current that is required to compensate for the
violation of the continuity equation in the impulse approximation. The second
is an exchange current, which arises in the quasipotential reduction from the
Bethe-Salpeter equation, and which represents effects of suppressed degrees of
freedom. Explicit general expressions are given for both of these exchange
currents. The results are applicable to both elastic and inelastic processes.
