The presence of a temporal order in living systems is manifested in biological rhythms. Attention was focused on the presence of diurnal and circadian rhythms. In view of the easy availability of leucocytes on the one hand and their involvement in numerous pathological conditions on the other, a study on the diurnal activity of six white blood cell (WBC) enzymes was performed in normal individuals. In one group of subjects the activities of four of the enzymes were measured twice a day for three consecutive days and in another group the enzyme activities were determined every 4 hrs for 24 hrs.