Incubation of rat-mouse T cell hybridoma cells, 23B6, with rat immunoglobulin E (IgE) results in the formation of the 15,000-dalton IgE-suppressive factor and the 30,000-dalton IgE-binding factor, which has neither potentiating activity nor suppressive activity on the IgE response. Another T cell hybridoma, 23A4 cells, produces the 30,000-dalton "inactive" IgE-binding factor upon incubation with IgE. Both the 15,000-dalton IgE-suppressive factor and the 30,000-dalton IgE-binding factor lacked affinity for lentil lectin but bound to peanut agglutinin. When the 23B6 cells were incubated with IgE in the presence of lysolecithin, the majority of the 15,000-dalton IgE-binding factor formed by the cells gained affinity for lentil lectin, and this factor selectively potentiated the IgE response. The glycosylation-enhancing factor, which was formed by stimulation of normal spleen cells with lymphocytosis-promoting factor (LPF or pertussigen), also switched 23B6 cells from the formation of IgE-suppressive factor to the formation of IgE-potentiating factor. It was also found that the 30,000-dalton "inactive" IgE-binding factor, formed by both 23B6 and 23A4 cells, gained the ability to potentiate the IgE response, when the cells were cultured with IgE in the presence of glycosylation-enhancing factor. The results indicate that IgE-potentiating factor and IgE-suppressive factor share common precursors, and that biologic activities of IgE-binding factors are decided by their carbohydrate moieties. Incubation of the two hybridoma cells with lysolecithin or glycosylation-enhancing factor results in an increase in the proportion of FC epsilon R+ cells, suggesting that the assembly of N-linked oligosaccharide to precursor molecules is intrinsic for the expression of FC epsilon R.