Serum IgM concentrations were measured in 25 patients with primary biliary cirrhosis (PBC) and 25 age- and sex-matched controls by the classical Mancini technique and by a modified method, which included reduction to 7 S monomers. Standards calibrated against the WHO standard, as well as a serum standard with an absolute value calibrated by immunological and non-immunological techniques, were used. All patients had an elevated IgM level; measurement of serum IgM is therefore a simple and sensitive screening test for patients with cholestasis. When measured with the standard calibrated against the WHO standard, the average IgM levels for PBC patients and controls agreed with results previously reported; the average IgM levels were much lower when a serum standard with an absolute IgM value was used; further standardization in needed. The differences in the ratio of the IgM measured by the classical method (pentameric IgM) to that measured by the alternative method (monomeric IgM) support the existence of different IgM subgroups or the in vivo presence of monomeric IgM in some patients with PBC.