The anti-DNP antibodies produced by primary and secondary splenic foci were analyzed for heavy chain class by a radioimmunoassay, using iodinated, purified goat antimouse µ-chain antibody and goat antimouse γ1 chain antibody. The frequency of primary and secondary foci producing both IgM and IgG1 anti-DNP antibody (16% and 14%, respectively) was considerably higher than that which would be predicted by a random distribution. It would thus appear that IgM and IgG1 antibody can be made by the clonal progeny of a single precursor cell
