Establishment of a mouse T hybrid cell line secreting suppressor factor(s) specific for the IgE antibody response is described. Fusion was made with polyethyleneglycol between AKR-derived T lymphoma cells (BW5147) and T cells from mice sensitized with DNP-Mycobacterium. Treatment of spleen cells with nondialyzable factor(s) in the culture supernatants of the T cell hybrid clone, 26-M10, showed a suppressive effect on IgE formation but not on IgG formation in adoptive transfer experiments. The suppressive effect was exerted through inactivation of normal or antigen-primed B cells responsible for IgE formation. It was also shown by direct cytotoxic test that the hybrid cells expressed H-2 and Thy-1 antigens derived from both parental cells on their surface. Karyotype analysis of the hybrid cells revealed that the number of chromosomes was less than the sum of the two parental cells' and the average was 50 (45 to 55). Although the 26-M10 hybrid cells lost the ability to secrete active suppressive factor(s) into culture medium 21 weeks after hybridization when the number of chromosomes in most of the cells was less than 41, recloning of the 26-M10 cells successfully recovered active suppressive clones.