A hybrid polypeptide is disclosed, comprising a polypeptide for attachment of a prosthetic group to avidin fused to at least one polypeptide of interest, and a method of making the same. The hybrid polypeptide is produced by recombinant DNA techniques, using a DNA expression vector composed of a DNA fragment coding for the polypeptide for attachment to avidin fused to DNA coding for one or more polypeptides of interest. The hybrid polypeptide may also contain linking amino acid sequences for cleavage of the polypeptide of interest from the polypeptide for attachment using an appropriate proteolytic or chemical reagent. The hybrid polypeptide is expressed in appropriate host cells transformed with the DNA expression vector encoding the hybrid polypeptide, and may be recovered from crude cell extracts in high yield and high purity using avidin affinity chromatography. Following avidin affinity purification, the polypeptide for attachment and polypeptide of interest may be cleaved to yield polypeptide of interest in a highly pure and highly active state.