A study was made of the effects of microwave irradiation of different intensity within decimeter and centimeter ranges in vitro on the guinea-pig thymocyte-induced receptor expression to their own and rabbit erythrocytes. Besides, effects of decimeter waves on mice thymocyte-induced expression of Thy-1 antigen were studied. Microwaves were found to modulate the thymocyte-induced expression of the membrane surface markers under study.