The clearance from the plasma of purified lactate dehydrogenase isoenzyme No. 5, which is increased in the plasma of mice infected with Riley virus, is impaired during the infection. That the clearance of purified isoenzyme No. 1 is unchanged by infection with Riley virus provides an explanation of why only the lactate dehydrogenase-5 isoenzyme activity is elevated in the plasma of infected mice.