The molecular heterogeneity of IgG antibodies to phosphocholine (PC) having a defined idiotype was examined in BALB/c mice immunized with PC-keyhole limpet hemocyanin (KLH). Specific antibodies were separated by isoelectric focusing in polyacrylamide gels and characterized for PC-binding, idiotype, and isotype by direct in situ labeling with 125I-labeled reagents followed by autoradiography. After immunization with PC-KLH, BALB/c produce 20 to 100 µg/ml of IgM and 80 to 300 µg/ml of IgG anti-PC antibody. The dominant fraction of anti-PC antibodies in BALB/c (and a lesser fraction in other strains) possesses idiotypic determinants found on a PC-binding myeloma, TEPC15. Among 65 BALB/c examined, all produced an identical spectrotypic pattern of antibodies possessing T15 idiotypic determinants. Three major sets of T15-idiotype bearing bands were observed, but they belonged to three different IgG subclasses: IgG1, IgG2, and IgG3. These data support the germ line origin for this dominant set of antibodies in the anti-PC repertoire of BALB/c and indicate that they arise from a single rather than multiple VH-VL pairs.