We have found that the myc oncogene has been modified by abortive recombination with the (Y heavy-chain immunoglobulin constant-region(C,) gene in five different mouse plasmacytoma lines. Recombination occurred approximately 0.8-2.0 kb to the 5’side of two distinct coding regions, defined by sequence homology between the chicken cellular and plasmacytoma myc genes. The myc and C, genes were always in opposite transcriptional orientation, with the recombination site within the C, switch region sequences. DNA recombination was found to correlate with the production of a novel 2.1 kb species of myc RNA that was 0.4 kb shorter than the normal cellular transcript. No elevated levels of myc RNA were evident, suggesting that DNA rearrangements have altered the myc oncogene product. This oncogene activation corresponds to the chromosomal translocations found in nearly all plasmacytomas.