Male CBA/J mice were exposed under far-field conditions in a temperature and humidity controlled environment to 2450-MHz (CW) microwaves. Mice were exposed once for 30 minutes at a power density of 15, 20, 30, or 40 mW/sq. mc. The whole-body-averaged dose rate was approximately 0.7 mW/g per mW/sq. cm. Six days after irradiation, the percentage of complement-receptor-positive (CR+) spleen cells was determined. No difference was observed in the percentage of CR+ spleen cells of young adult (10-12 week-old) mice exposed at any of the power densities as compared with sham-irradiated controls. However, a significant (P<.05) increase was observed in the percentage of CR+ cells from 16-week old mice exposed at 40 mW/sq. cm. This increase in CR+ cells was accompanied by a significant (P<.05) decrease in the number of nucleated cells in the spleens of these mice. This change in CR+ and nucleated spleen cells was not consistently produced. The available data indicate that the age and strain of the mouse, the microwave exposure characteristics, and the environmental conditions may all be sources of variation that affect the CR+ end point.