The presence (in sera of rhesus monkeys) of four antigen-specific molecules with different electrophoretic mobilities apart from conventional immunoglobulins has been recognized. Rhesus monkeys have been primed with rabbit erythrocytes (RRBC) and the antigen-specific molecules in the serum have been fished out by absorption onto RRBC. Antisera have been raised in rabbits against such molecules and analysed by immunoelectrophoresis. The antigen-binding property of the four molecules, which do not resemble conventional immunoglobulins has been confirmed by enzyme immunoelectrophoresis. The possibility of these molecules being products of T lymphocytes in the light of observations reported in recent years is discussed.