The low affinity IgE receptor (Fc epsilon RII/CD23) has been proposed to be involved in the regulation of IgE synthesis. The present study was undertaken to investigate the responses to in vitro stimulation by allergen (Dermatophagoides pteronyssinus; D.p) and/or interleukin-4 (IL-4) of peripheral blood lymphocytes (PBLs) isolated from atopic and non-atopic subjects. IL-4 induced up to 5 fold increase in CD23 expression on PBLs from both atopic patients and normal controls, whereas the D.p extract increased CD23 expression on cells from 7 of 8 atopic donors and from 2 of 8 normal controls. The combination of IL-4 and allergen had an additive effect of CD23 expression. PBLs from 6 of 8 atopic patients but 1 of 8 normal controls showed significant proliferative responses to D.p extract whereas IL-4 did not induce any cell proliferation. The dose of D.p extract required for the maximal CD23 expression was 20 fold higher than that for cell proliferation. These results imply that allergen stimulation, presumably through proliferating allergen specific T cells which secrete IL-4, activates B cells from most atopic donors and a few non-atopic donors resulting in increased CD23 expression. This allergen-mediated CD23 expression may play an important role in specific IgE production.