The IgG fraction of goat anti-mouse IgM (μ-chainspecific) antiserum is covalently attached to cyanogen bromide activated cross-linked beaded agarose. A minimum of five milligrams of the IgG fraction is bound per milliliter of resin. After equilibration, ≥0.4 mg of mouse IgM can be bound and eluted per milliliter of packed resin.