<p>A method of quantifying an ethylene oxide-based nonionic surfactant in a sample containing a physiologically active protein characterized by using colorimetry for the quantification. In particular, a method of quantifying an ethylene oxide-based nonionic surfactant in a sample containing a physiologically active protein characterized by comprising adding an ammonium thiocyanate/cobalt nitrate reagent (CoSCN reagent) to a solution of the sample and quantifying the thus formed cobalt thiocyanate complex by absorption spectrometry.</p>
