Manufacturing of biological therapeutic agents such as growth hormone, tissue plasminogen activator, alpha interferon, and PEGylated proteins requires new analytical methodologies for characterization and quality control of the products. A new methodology based on the technique of partitioning in aqueous polymer two-phase systems is recommended for quality control testing. The advantages of the methodology are that it provides unique information that is related to the biological potency, and it is rugged and simple. Here we describe the basics of the technique and the quantitative information it provides and give examples of how the technique can be applied for characterization and quality control of biological and recombinant products.