Supernatants derived from T cells of donors with and without atopic disease were assessed to determine whether they could regulate ongoing IgE, IgG, or IgA synthesis of three different human B-cell lines. The results were compared with the ability of T-cell supernatants from the atopic donors to specifically induce IgE synthesis of nonatopic peripheral blood B lymphocytes (PBL-B cells). We found that, when tested on the B-cell lines, the T-cell supernatants from atopic donors doubled the IgE as well as the IgG and IgA synthesis. Although the T-cell supernatants from nonatopic donors had no effect on Ig synthesis of PBL-B cells or on the IgG of the IgG-secreting cell line, the supernatants doubled the IgE and moderately enhanced IgA synthesis of the IgE- and IgG-secreting cell lines. These results demonstrate that T-cell supernatants derived from atopic and nonatopic donors differ in their regulatory effects, not only on PBL-B cells, as had been shown previously by others, but also in their effects on ongoing Ig production.