The warming properties of the Taurus radiowave blood warmer were studied. The safety limits were the same as for microwave warming (Haemotherm). Hemolysis was noted in units with a hematocrit over 0.75 or containing less than 300 g of blood. The effects of radiowave, microwave and water bath warming on the erythrocyte ATP and 2,3 DPG concentrations were compared. Statistically highly significant differences were found between all these methods. Warming of blood in a +35 degrees C water bath for 3 min had no effect on these parameters. There was, however, a tendency for ATP and 2,3 DPG levels to decrease after 60 min water bath incubation (+37 and +45 degrees C), but to increase levels following electromagnetic warming depending on the age of the warmed erythrocytes. Thus, electromagnetic radiation appeared to influence the red cell metabolism, which cannot be explained by the effect of temperature alone.