A method has been developed for assay of apolipoproteins A, B and C in human serum, using rocket immunoelectrophoresis. The technique is reasonably simple, sensitive and precise. The concentrations of apolipoproteins A, B and C in these classes of serum lipoproteins were measured in 116 healthy subjects. Concentrations of apoproteins within very low and low density lipoproteins increased with age but those of high density lipoproteins showed no consistent age trend.