Oligosaccharide chains on the surfce of human erythrocytes were labeled with the probe eosin 5-thiosemicarbazide. The probe was conjugated to aldehydes produced by oxidation of sialic acid and galactose residues. The probe is associated mostly with glycophorin A after sialic acid labeling, whereas multiple components, including band 3 and lipids, are labeled after galactose oxidation. Fast molecular motion was studied by measuring steady-state and picosecond time-resolved fluorescence depolarization. Slower motions were investigated by observing flash-induced transient dichroism. It was found that both eosin-labeled sialic acid and galactose residues exhibit a rapid motion with correlation time of approximately 3 nsec. This motion is assigned to independent motion of the probe, possibly in conjunction with a short segment of the oligosaccharide chain. The order parameter of the fast motion is 0.8.-0.9, demonstrating that its angular amplitude is highly restricted. For eosin-labeled sialic acid, the order parameter in the microsecond time range is 0.2-0.3. It is deduced that a second, slower rotational motion is present, which is assigned to a cooperative motion of the oligosaccharide chains. The correlation time of this motion is in the range 10(-7)-(10-5) sec. Some eosin-labeled galactose residues may have a similar slow motion, but most appear to be remarkably immobile over the time range 10(-8)-10(-3) sec.