Burkitt's lymphomas (BLs) and mouse plasmacytomas (MPs) are characterized by chromosomal translocations juxtaposing the c-myc oncogene to one of the immunoglobulin loci. Here we describe illegitimate recombinations and the DNA sequence at the junction between c-myc and immunoglobulin heavy chain (IgH) regions in these B-cell lineage tumors. In 15 BLs and in 29 MPs, recombination products were amplified by PCR and sequenced. Molecular analysis of the IgH/c-myc junctions revealed the following characteristics: (a) rearrangements in BLs represent nearly reciprocal exchanges between c-myc and IgH switch sequences, whereas in MPs, several hundred bp of c-myc may be duplicated or deleted; (b) short homologies of up to 5 bp between immunoglobulin and c-myc sequences can be found at most junction sites in both tumors; and (c) pentameric sequences typically found in switch regions and V(D)J recognition motifs also occur frequently at the sites of recombination in c-myc.