We have reviewed studies of a cross-reactive idiotype (CRI), associated with anti-para-azophenylarsonate (anti-Ar) antibodies of A/J mice, and have presented some recent data. H chains from molecules with the CRI are homogeneous through the first hypervariable region whereas L chains show heterogeneity in the first framework segment. The constancy of N-terminal H chain sequences from 10 individual mice indicates that the framework is encoded by a germ-line gene, and that mutations have little effect on the observed framework sequence. H chains lacking the CRI show great heterogeneity in the first hypervariable segment, indicating a direct relationship between sequence and idiotype. Adult A/J mice can be suppressed with respect to the production of the CRI by administration of rabbit antiidiotypic antibody. When immunized, such mice produce anti-Ar antibodies with "private" idiotypes present at extremely low concentrations, or absent, in anti-Ar antibodies of other A/J mice. These private idiotypes can be transferred adoptively to mildly irradiated (200 r) syngeneic recipients with lymphoid cells, and serial transfers can be carried out after immunization. The original CRI remains suppressed. This presents the possibility of structural studies of anti-Ar antibodies varying in idiotype from a single strain. The work should also provide information on the degree of diversity of anti-Ar antibodies within the strain.