Rabbit erythrocytes were exposed in vitro to continuous wave (CW) and pulse-modulated X-band microwaves in wave guide exposure chambers. Erythrocytes were exposed as whole (heparinized) blood suspensions or as washed cells in 1:1 isotonic buffered K+-free saline suspensions. Statistically significant increases in K+ efflux relative to thermal controls were detected when red cells were exposed in whole blood suspensions to either CW or pulsed 8.42-GHz microwaves at SARs that resulted in equilibrium sample temperatures of approximately 24 degrees C. Under the same exposure conditions, no statistically significant K+ efflux occurred in the case of 1:1 red cell suspensions. Measured differences in sample heating rates and temperature gradients between microwave-exposed and heated control suspensions may account in part for the differential effect of microwave exposure but such effects do not appear to explain the results of this study fully.