Abstract A technically simple procedure is described for simultaneous determination of numbers and functions of human blood lymphocyte subpopulations. The method is based on stimulation with mitogens (PHA, Con A and PWM) with simultaneous rosetting to detect E receptors and C3 receptors. Con A and PWM activate the affinity of C3 receptors in normal lymphocytes increasing the number of cells rosetting with zymosan-complement particles, and also the number of cells giving mixed rosettes with both zymosan-complement particles and sheep red blood cells (B cells and ‘D’ cells respectively). PHA activates E receptors, increasing the number of T cells detected. The lymphocytes of 33 patients with allergic diseases of the skin and respiratory tract showed a decrease in C3 rosettes during stimulation with Con A and PWM, which suggests a defect of helper and suppressor functions of lymphocytes in these patients.