The effect of dexamethasone (DM) on de novo in vitro total IgE synthesis by blood mononuclear cells (MNC) was studied in atopic patients with eczema and in nonatopic control subjects. Unfractionated blood MNC were cultured at 1 X 10(6) cells per milliliter for 7 days in RPMI 1640 with 10% fetal calf serum with or without decreasing concentrations of DM (10(-7) to 10(-11). Net IgE synthesis was calculated by subtracting preformed (+ cycloheximide at day 0) from total IgE in 7-day supernatants. Supernatant IgE was measured by use of a modified PRIST assay. A significant increase in net IgE synthesis occurred in the presence of DM in 11 of 11 atopic patients with eczema (mean percent increase = 68%; p less than 0.05) and five of five atopic patients without eczema (mean percent increase = 53%; p 0.05) but not in seven of seven nonatopic controls. This increase in de novo IgE synthesis could not be explained by a significant change in cell viability. In five of five experiments, a mean increase of 78% was still noted when DM was added to atopic blood MNCs depleted of T cells by sheep red blood cell rosetting. The addition of 10(-9)M of DM to eczema B+ T cell recombinations enriched for suppressor cells (depleted of Leu 3a+ helper T cells) resulted in a loss of suppressor-T cell activity and maximal augmentation of IgE synthesis. Enhancement of IgE synthesis was also noted when DM was added to eczema B+ T cell recombinations enriched for helper T cells (depleted of suppressor Leu 2a+ T cells).(ABSTRACT TRUNCATED AT 250 WORDS)