A fraction of serum anti-PC antibodies from mice immunized with PC-KLH expresses idiotypic determinants of the PC-binding myeloma MOPC-511. We examined the molecular heterogeneity of IgG antibodies bearing the M511 idiotype (Id) by analytical isoelectric focusing followed by in situ labeling with 125I-anti-M511. Antibodies with the M511 Id were shown to be conserved in many mouse strains and were distinct from T15 Id+ anti-PC antibodies. Two complex sets of M511 Id+ bands were observed and these were identified as M511 Id+ antibodies belonging to the IgG1 and IgG3 subclasses. Examination of responses in different individuals of a strain permitted identification of at least 4 to 6 IEF band groups. Thus, the M511 Id is a marker for a family of closely related molecules. Quantitative variations were observed in the expression of the M511 family within a strain and between strains of mice that occurred independently of expression of the dominant T15 Id+ fraction. These results support the concept of an M511 Id family and further suggest that there can be differential control mechanisms governing expression of individual clones or a family of closely related clones.