Human erythrocytes were isolated and incubated in various media at constant and light‐dark‐conditions. Within these suspension cultures the activities of the tightly membrane‐bound enzyme acetylcholinesterase and of the partly bound enzyme pyruvate kinase, were measured during 24 hours. Membrane dynamic structure was investigated during the same time interval by means of the valinomycin‐facilitated potassium transport, whereas membrane fluidity was determined using the fluorescence depolarization method. In all cases considerable changes during the incubation time were registered, some of which showed a 9 h‐, 12 h‐, or 24 h‐periodicity. However, it was not possible to arrive at statistically significant periodicities after averaging the individual experiments, except that in some single experiments the acetylcholinesterase activity seemed to be rhythmic. This result may be due in part to the different phasing of individual red blood cell suspensions.