Human blood platelets in platelet-rich plasma were irradiated at room temperature with 2.45 GHz continuous wave microwave radiation in a rectangular waveguide chamber for an exposure duration of two hours. Measurements of the change in platelet count, release of lC-radiolabeled serotonin, increase in the level of LDH activitiy in the supernatant platelet-free plasma, alteration in the aggregation response to exogenous ADP, and decrease in the intracellular potassium ion concentration were assayed to determine any damage to the platelets. In all experiments, microwave heated platelets were compared with platelets warmed to the same temperature by use of an adjustable block-heater and with platelets maintained at room temperature. Preliminary studies indicated that the incident power flux was not a crucial factor for the power fluxes considered, 3.35 to 2.12 mW/cm^2. The subsequent experiments were performed at the maximum power flux of 2.12 mW/cm^2, which corresponds to an absorbed power of 112.62 mW/cm^2. At this power flux, the final temperature was 41°C. There were no significant differences between the block-heated and microwaved samples in the release of serotonin, the increase in LDH activity, and the decrease in intracellular potassium ion concentration . Significant differences were observed between the block-heated and microwaved samples in the platelet count and in the aggregation response to exogenous ADP. However, these observed differences were small, and there was more alteration of platelet response from block-heating than from irradiation. Therefore, it may be concluded that the observed response of human blood platelets exposed to 2.45 GHz continuous wave radiation at a power level of 112.62 mW/cm^2 is entirely due to temperature elevation caused by energy absorption.