The lymphocyte membrane fluidity of patients with allergic diseases was measured by electron spin resonance (ESR), and the effect of the epinephrine stimulation on the membrane fluidity was examined. The peripheral lymphocytes were obtained from 15 patients with bronchial asthma and atopic dermatitis (28.7 +/- 9.9 years old, 10 females and 5 males) and 11 healthy adults (30.5 +/- 4.6 years old, 2 females and 9 males). Lymphocyte membranes were spin-labeled with 5-doxyl-stearic acid. Before and after the stimulation of epinephrine of the final concentrations at 10(-5) and 10(-4) mol/l, ESR spectra of the outer membranes were analyzed to evaluate the membrane fluidity. The membrane fluidity of the intact lymphocytes of allergic patients was significantly decreased in comparison to healthy controls. Although the epinephrine stimulation increased the lymphocyte membrane fluidity, the increase in fluidity was less in allergic patients than in healthy controls. There are various receptors on the surface of the lymphocyte membranes, and changes of the membrane fluidity have an influence on their functions. The results in this study elucidate the decreased fluidity of lymphocyte membrane in patients with bronchial asthma and atopic dermatitis, and suggest that the functions of the membrane receptors might be impaired.