Different recombinations have been found at the Ig heavy chain gene loci in a number of sublines of the Burkitt lymphoma (BL) cell line Namalwa, following prolonged in vitro culture. The Namalwa sublines examined are DNA fingerprint-identical and derived from a monoclonal source. Recombinant DNA clones were used to map the Ig heavy chain gene mutations to a region between the VDJ and C mu segment of the locus. This region is associated with Ig heavy chain class switching in normal B cells. Of 24 clones established from one subline, three were found to have additional VDJ-C mu region mutations, indicating a high frequency of mutation at this locus.