Double fluorescence studies indicated that most mature lymphocytes of 11-52-wk-old mice possess both IgM and IgD-like surface immunoglobulins, while spleen cells from neonatal mice possess surface IgM only. These molecules cap independently with class-specific antisera, but co-cap when capping is induced by antigen. It is proposed that the two heavy chains on individual lymphocytes possess similar or identical antigen-combining sites.

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