There is an increase in the number of CD23+ cells in peripheral blood mononuclear cells (PBMC) in atopic dermatitis (AD). We analysed the subpopulation of CD23+ PBMC in 11 patients with AD and in 10 healthy controls and found that B cells (CD20+) and non-T, non-B cells (CD3- CD20-) (mainly monocytes) were responsible for the elevation of CD23+ cells. CD23+ T cells (CD3+) comprised only 4.6% of total CD23+ cells in AD. The percentage of CD23+ cells did not correlate with the serum log IgE level nor with clinical severity of AD. Interleukin 4 (IL-4) induced the expression of CD23 antigen in PBMC both in AD and in healthy controls in a dose-dependent manner in vitro. This enhancing effect of IL-4 was completely abrogated by the addition of anti-IL-4 monoclonal antibody. Other cytokines such as IL-1, IL-2, IL-3, IFN-alpha, IFN-gamma and TNF-alpha had no significant effects on CD23 expression.