In vitro IgE synthesis by blood mononuclear cells from atopic patients and nonatopic subjects was examined. A total of 1 X 10(6) mononuclear cells cultured in RPMI-1640 and 10% fetal calf serum with or without cycloheximide was found to be optimal to detect de novo synthesis. A modified Phadebas IgE paper radioimmunosorbent test was employed for the quantitation of supernatant IgE concentration. Kinetic studies indicated that about half the peak amount of IgE is secreted within the first 2 days and the maximum concentration is reached at day 7. Mononuclear cells obtained from six of six atopic patients with eczema and elevated serum IgE levels and 22/33 atopic patients without eczema spontaneously synthesized significant amounts of IgE in vitro. We failed to detect de novo IgE synthesis by the cells obtained from 40 nonatopic controls. Polyclonal activators such as pokeweed mitogen. Staphylococcus aureus Cowan I, concanavalin A, and phytohemagglutinin failed to induce or enhance in vitro IgE synthesis in normal and atopic subjects. These findings indicate that the study of immunoregulation of IgE synthesis in man will be difficult to accomplish until new methods are developed that allow induction of the IgE response in vitro in nonatopic subjects.