In allergic patients the authors previously observed high proportions of circulating T lymphocytes bearing Ia antigens, assumed to be "activated" T cells. In the present investigation they employed other T cell activation markers (4F2, insulin receptor, MLR4) which differ in the kinetics of appearance upon the surface of stimulated T cells. They report high proportions of Ia and 4F2-positive T cells, normal levels of MLR4-positive T lymphocytes and no insulin binding on T cells. However, T cells of allergic subjects are able to express insulin receptors in PHA-induced culture, such as normal subjects do. The authors conclude that these data, supported by similar observations in autoimmune diseases, indicates differences between in vivo and in vitro features of expression of T cell activation markers. In addition the autologous mixed lymphocyte reaction (AMLR) in atopic patients was studied. The results indicate that AMLR responsiveness is defective in allergic patients.