Circadian variations in the phagocytic activity of mouse whole blood, spleen, and peritoneal cells were studied using the zymosan-induced chemiluminescence assay as a measure of phagocytosis. On a regimen providing for light from 7:00 to 19:00 alternating with darkness, the phagocytic activity of mouse blood, spleen, and peritoneal cells was high around 10:00 and low around 22:00, the integrated counts of chemiluminescence being 82.33 x 10(5) and 52.76 x 10(5) for peritoneal cells, 83.3 x 10(5) and 32.2 x 10(5) for spleen cells, and 12.33 x 10(5) and 3.99 x 10(5) for blood cells. Variations of a similar tendency were also found in blood leukocyte and granulocyte counts, the counts being again higher at 10:00 compared with the blood samples withdrawn at 22:00. In contrast to the differences in the intensity of the zymosan-induced chemiluminescence, the shapes of the curves (Fig. 6) of each cell preparation were similar, irrespective of the time period of the day the cells were prepared. Comparison of the zymosan-induced chemiluminescence curves of the 3 cell suspensions studied, prepared at the same period of the day, revealed some similarity between the kinetics of blood and spleen samples; the intensity, however, of zymosan-induced chemiluminescence emitted by spleen cells was much higher. The kinetics of zymosan-induced chemiluminescence curves of peritoneal cells differed from the other 2, being slower at the onset, the chemiluminescence lasting for a longer time and declining more slowly. We have shown here circadian variations in the activity of mouse phagocytic cells. The simple and rapid method of chemiluminescence measurements used in this study appears to be a powerful tool for the further investigation of such circadian variations.