The Burkitt's lymphoma cell lines JBL2 and Ly91 contain intracytoplasmic mu chains without L chain by immunofluorescence. These mu chains and the corresponding cDNA are abnormally short. Detailed analysis of the productive H chain alleles showed that the cell lines JBL2 and Ly91 use a VHIV and a VHIII subgroup gene, respectively. In both cell lines, the 3' part of the V exon was markedly abnormal and contained several stop codons. Point mutations affected the JH6 donor splice site in the JBL2 gene, precluding normal splicing into C mu. Although the JH2 splice site was unmodified in Ly91, it was not used. As in JBL2, splicing occurred at an alternate splice site present 5' of these alterations, thus removing all these abnormalities from the mature transcripts. This alternate splicing explains the presence in both cell lines of truncated mRNA and proteins lacking two-thirds of the V region.