A method for the isolation of intact U5 small nuclear RNP particles from HeLa cells has been developed. The procedure includes nuclear extraction of the particles at moderate ionic strength, fractionation in agarose gels, electrophoretic transfer on DEAE cellulose paper and elution with ammonium chloride. The purified U5 snRNP particles contain U5 RNA and a set of eleven proteins. They retain their antigenicity as judged by their reaction with autoantibodies from patients with connective tissue diseases.