Soluble CD23 (sCD23) is increased by interleukin-4 (IL-4) and decreased by interferon-gamma (IFN-gamma). On the basis of cytokine profiles T-helper (Th) cells may be functionally divided into IL-2- and IFN-gamma-secreting Th1 cells, which are involved in cell-mediated immunity (CMI), and IL-4- and IL-5-producing Th2 cells, which are involved in humoral immunity. Compared with sex-matched controls (median 8.5) we found significantly elevated levels of serum sCD23 in patients with rheumatoid arthritis (median 22.7, P < 0.0002), with the highest levels detected in patients fulfilling an increasing number of the American Association revised criteria for rheumatoid arthritis. Soluble CD23 levels were also significantly raised in autoimmune thyroiditis (median 11.7, P < 0.02) and myasthenia gravis (median 10.4, P < 0.05). In contrast patients with either coeliac (median 6.5) or Crohn's disease (median 5.8) had reduced levels of sCD23 compared to appropriate controls (median 11.8), in both cases significant at P < 0.01. Variations in sCD23 may, therefore, reflect enhanced Th1 activity in the two later conditions in contrast to heightened Th2 activity within the three classical autoimmune conditions.