One M-component is described with anti-γG and anti-dinitrophenyl and anti-trinitrophenyl activity. The anti-hapten activity was shown by precipitation of DNP– and TNP–protein conjugates; this precipitation was specifically inhibited by the DNP–hapten. All of the M-component protein had these anti-hapten activities. The whole M-component also precipitated aggregated γG. DNP inhibited the anti-γG activity in precipitation and agglutination systems. Rheumatoid factors from sera of patients with classical rheumatoid arthritis exhibited similar, although weaker, reactivity with the haptens. The data indicate that rheumatoid factors cross-react with dinitrophenyl and trinitrophenyl groups.