An amino acid sequence analysis of the N-terminal immunoglobulin heavy and light chain variable regions (V_H and V_L) from 16 hybridoma proteins which bind phosphorylcholine as well as the complete sequence analysis of 9 of these V_H regions is presented. There seem to be more V_H regions participating in the phosphorylcholine response than can be encoded directly by germ-line V_H gene segments. Moreover, the V regions from IgG antibodies are considerably more variable than those from their IgM counterparts. These observations raise the possibility that a somatic mechanism for V region diversification produces greater diversity in IgG than in IgM antibodies.