Rabbits were immunized with group A-variant (Av) streptococcal vaccine to elicit antisera containing antibodies of "restricted heterogeneity" specific for Av polysaccharide. Antibodies that migrated as single bands in polyacrylamide gel isoelectric focusing were isolated on agarose block electrophoresis and preparative isoelectric focusing. These homogeneous antibodies were analyzed for content of the a locus markers by inhibition of allotype-anti-allotype radioimmunoassays. Both single-band preparations and preparations containing 2--5 bands were deficient in content of the a locus markers. These analyses were strongly influenced by the different anti-allotype antisera used. Single-band preparations, deficient in a 2 or a 3 markers, were further used to produce anti-allotype antisera, and these sera were assayed for reaction with pooled a2 or a3 molecules. The results, in contrast to the a1 allotype, failed to demonstrate common determinants on either a 2 or a 3 molecules indicating that each a allotype consists of a set of different Ig molecules. The data suggest different evolutionary pathways of the a 1, as opposed to the a 2 and a 3 molecules.