The lymphocyte subpopulations were classified using monoclonal antibodies specific for B lymphocytes (B1 antibodies), T lymphocytes (T11 and OKT3 antibodies), helper/inducer T cells (T4 antibodies) and suppressor/cytotoxic T cells (T8 antibodies). Three groups of subjects were studied: 20 normal controls, 29 patients with allergic rhinitis and a subgroup of nine patients who had received immunotherapy. The proportion of B lymphocytes, total T cells and T4 positive (helper/inducer) cells were not significantly different between the groups, but allergic patients were found to have a decreased proportion of suppressor T8 positive (suppressor/cytotoxic) cells and hence a high helper/suppressor cell ratio. These abnormal parameters were found to be normal in the group of allergic patients who had received immunotherapy. These results imply that a suppressor cell deficiency may be an underlying mechanism of allergic disease, and that immunotherapy could correct the suppressor cell deficiency.