Blood lymphocyte subpopulations were investigated in 50 patients with allergic diseases (38 with allergic rhinitis and 12 with allergic bronchial asthma). The total T lymphocyte (Tt) count (E rosette test), the active T lymphocyte (Ta) count (active E rosette test) and the B lymphocyte count (EAC rosette test) were determined and the results were correlated with: 1) lymphocyte sensitization in vitro to different allergens and to PPD; 2) circulating immune complexes; 3) serum histaminopexic capacity. In allergic rhinites a decrease of Tt and B lymphocytes and an increase of Ta lymphocytes was observed. In allergic bronchial asthma the Tt lymphocyte were found reduced. Lymphocyte reactivity to PPD in vitro was present in 100% of the cases and in vitro lymphocyte sensitivity to several allergens was observed in most of the cases. The histaminopexic capacity was absent or low in 75% of the patients with allergic rhinitis and in 66.6% of the patients with allergic bronchial asthma. The involvement of cellular mediated immunity in allergic diseases is discussed.