The nucleic acid sequence of the heavy chain variable region (VH) expressed by 38C13, a B cell tumor of C3H origin, was determined by a combination of direct (messenger RNA) mRNA sequencing by primer extension and complementary DNA (cDNA) isolation and sequencing in M13. The VH amino acid sequence was deduced, and hypervariable regions were identified. From an analysis of predicted secondary structure, regions of predicted antigenicity were chosen, and a series of synthetic peptides corresponding to CDR2 and CDR3 (complementarity-determining region) were produced. These peptides were coupled to protein carriers and used to immunize syngeneic C3H mice. All peptides gave rise to a vigorous antibody response. However, only the CDR3 peptides induced antibodies that crossreacted with the isolated H chain protein. Only one CDR3 peptide induced antibody-producing clones, isolated as hybridomas, that reacted with the intact IgM protein. However, the appearance of these clones was a low-frequency event. All antibodies reacting with the H chain or the intact IgM protein were idiotypically specific for 38C13. These monoclonal antiidiotype (anti-Id) antibodies, raised against CDR3 peptides, gave strong reactions in enzyme-linked immunosorbent assays and immunoblots, but they were of low affinity compared to syngeneic anti-Id raised against the intact IgM protein. Moreover, while the intact IgM was capable of inducing tumor immunity, the CDR peptides were not able to do so.