We describe in this paper the rapid and cheap purification of human immunoglobulin M from hybridoma supernatant of mass culture. The method consists of two steps: 1. concentration of supernatant by ultrafiltration and 2. dialysis against distilled water, pH 6.4. To produce the supernatant, the hybridomas are grown in RPMI media with fetal calf serum (10% FCS) in 250 ml flasks under normal tissue culture conditions. More than 14 mg IgM can be nearly selectively purified within 6 h from 5 l of antibody containing hybridoma supernatant. Most of the IgM molecules stay in a penta- and monomeric form and are positive in physiological and immunohistochemical studies.