Concanavalin-A-activated T cells and their crude supernatants were assayed for suppressive activity on an IgE-producing U-266 cell line. Detectable and comparable degrees of suppression were obtained with the co-culture and the supernatant protocols. Separation of the effector population into T4+ and T8+ subsets showed the most effective cells in the T8+ fraction. Control experiments demonstrated that the IgE down-regulation was selective, since parallel measurement of beta 2-microglobulin synthesis showed no effect of T cells or T-cell-derived supernatants. In addition, several human T-cell lymphoma-leukaemia virus I-transformed T-cell lines were explored for their capacity to produce factor(s) able to suppress IgE synthesis in the U-266 cell line, and four out of 25 cell lines could be shown to do this in a constitutive manner. Kinetic studies suggested that the inhibition occurred at a transcriptional level. The results indicate that the T-cell-myeloma system is an interesting model to define better the regulation of IgE in the human.