Rabbit anti-human IgG antibody was mildly reduced in the presence of 10 mM 2-mercaptoethylamine and then coupled to β-D-galactosidase [EC 3. 2. 1. 23] from Escherichia coli using N, N'-o-phenylenedimaleimide. Human IgG and the anti-human IgG antibody-β-D-galactosidase complex were successively adsorbed on Sepharose 4B binding rabbit anti-human IgG antibody. In this way amounts of human IgG as small as 5×10−15 moles could be measured by determining the activity of β-D-galactosidase bound to the Sepharose.