Limited papain hydrolysis of immunoglobulin M (IgM) produces a subunit-like proteolytic fragment designated IgM p (Inman & Hazen, 1968). In the presence of mercaptans, IgM p partially dissociated into Fc μ -like and Fab μ fragments. Treatment of residual IgM (that remaining after a papain digestion) with 2m m -mercaptoethylamine resulted in fragmentation of the same type that occurs in a routine limited digestion of IgM with papain, although exogenous enzyme was not added to the mixture. When IgM was hydrolysed with 14 C-labelled papain, a small quantity of the enzyme was found to be associated with the residual IgM and IgM p fractions. IgM and IgM 7S subunit (IgM s ) that had been exposed to papain in the absence of activating mercaptan and separated from the enzyme by gel filtration also fragmented when subsequently treated with 2m m -mercaptoethylamine. The fragments resembled those produced during a typical limited papain digestion of IgM. It was concluded that mercaptoethylamine induced fragmentation of IgM p by activating adsorbed papain.