Human erythrocyte actin (band 5) was purified by acetone powder extraction, repeated F-G exchange and gel filtration on a Sephadex G-150 column in the presence of sodium dodecyl sulfate (SDS). The molecular weight of the purified erythrocyte actin was determined to be 43, 500 by sedimentation equilibrium. Isoelectric focusing gel analysis showed that the purified erythrocyte actin existed in the β-form with an isoelectric point between those of rabbit skeletal muscle α-actin and chicken gizzard γ-actin. The purified erythrocyte actin was found to be very similar to other actins in amino acid compositions and in the electrophoretic analysis after the CNBr-degradation.