We previously described human T cell leukemia/lymphoma virus-1-transformed T cell clones expressing surface molecules binding to monoclonal antibodies against lymphocyte Fc epsilon receptor (Fc epsilon R) and releasing soluble factors binding to both IgE and to monoclonal antibody to lymphocyte Fc epsilon R. In this study, one such clone (HE1-11) was tested for the presence of mRNA hybridizing with a cDNA probe coding for Fc epsilon R on B cells. Northern blot analysis revealed the presence of the same 1.7 kb Fc epsilon R mRNA in HE1-11 and in RPMI 8866 cells as well as in the macrophage cell line U937, expressing Fc epsilon R. The IgE binding factors (BF) released by HE1-11 cells appeared to be identical to those released by RPMI 8866 cells as shown by 1) one- and two-dimensional sodium dodecyl sulfate/polyacrylamide gel electrophoresis analysis and 2) tryptic peptides analysis. IgE-BF are microheterogeneous containing molecules with molecular mass of 25 to 27 kDa and with isoelectric point ranging from 4.5 to 5. Some preparations further contained 16-kDa fragments of IgE-BF. These findings suggest that the gene coding for both Fc epsilon R and IgE-BF may also be expressed on some human T cells.