A human IgE-producing myeloma has been cultivated in vitro as a continuous cell line (U-266) since 1968. Analysis of immunoglobulin production during early passages of the cell line demonstrated a high synthesis rate of monoclonal IgE. Analysis of late passages, cultivated after 1980, revealed a 3-6-fold lower rate of IgE secretion. This decrease was accompanied by the appearance of small amounts of IgA in the culture medium together with IgE. RNA was extracted from a late passage of U-266 and analyzed by Northern blotting, using epsilon and alpha-specific oligonucleotides as hybridization probes. The results showed the presence of epsilon as well as alpha-specific mRNA. Moreover the results demonstrated that the latter mRNA was derived from the alpha 2 locus and that the epsilon and the alpha 2-specific mRNA contained the same V region sequences. Southern blot analysis of DNA from the late passage of the U-266 cell line failed to reveal a recombinatory switch from the epsilon locus to the alpha 2 locus. The expression of alpha 2 is thus likely to be caused by differential splicing rather than by an isotype switch at the DNA level.