The effect of nonionizing electromagnetic radiation in the microwave range was studied on the adherence, opsonization, phagocytosis, nitroblue tetrazolium reduction and antibody-dependent cell cytotoxicity of peritoneal macrophages from BALB/c mice. After 15 min of irradiation with microwaves modulated in pulses at a frequency of 2,450 MHz and a power density of 32 mW/cm2, an increase in all the parameters studied was observed. The use of microwaves as a possible tool for the stimulation of the immune system is discussed.