The status of the immunoglobulin (Ig) genes was investigated in patients with idiopathic nonmalignant monoclonal IgG cryoglobulinemia (NCG). In NCG, monoclonal antibodies are synthesized at an accelerated rate by nonmalignant B lymphocytes. In order to determine whether this high production rate is related to a clonal B cell expansion, the rearrangement of the Ig genes was investigated by Southern blot analysis of genomic DNA extracted from the peripheral blood lymphocytes of four NCG patients. In three of four (VI, BR, and CH) clonal expansion of B cells was detected using probes specific for the c kappa, JH, c gamma 4 genes (in accordance with detecting IgG kappa cryoproteins in these patients). BamHI digestion of DNA from VI and BR produced three rearranged fragments which cohybridized with JH and c mu probes. This finding suggested the presence of additional nonsecretory B cell clones and/or disruption of the gene segments spanned by and detected with the probes. In VI the idiotype of the IgG cryoglobulin was also detected in association with IgM in the supernatant of Epstein-Barr virus-stimulated B lymphocytes using a murine monoclonal anti-idiotypic antibody. In addition, the possibility of aberrant gene rearrangements was supported by noting the alteration of the c-myc gene locus in genomic DNA from peripheral blood leukocytes of VI and CH. Northern blot analysis of RNA isolated from peripheral blood B cells of VI and CH demonstrated aberrant transcripts of the c-myc gene, showing an active role of the altered c-myc locus. Detection of c-myc rearrangement in NCG patients clearly shows that this event may not be a final step in malignant B cell transformation; however, it may be related to the clonal expansion and high rate of cryoglobulin synthesis of nonmalignant B lymphocytes.