Anti-phosphorylcholine specificity has recently been shown to occur with relatively high incidence among IgA myeloma proteins secreted by oil-induced plasma cell tumors in the BALB/c strain of mice. A similar screening of human myeloma sera indicates that in man activity for phosphorylcholine is very rare. Among 904 human sera containing IgG, IgA, and IgM M-components only one reacted with phosphorylcholine-containing antigens. This serum was obtained from a patient with macroglobulinemia Waldenström. The active homogeneous protein could be isolated by affinity chromatography using a Sepharose-phosphorylcholine immunoadsorbent. It was an IgM immunoglobulin; the light chains were of the kappa type. The association constant for the reaction with phosphorylcholine was homogeneous and equalled 6.4 times 10-4 l. mol-1 at 25 degrees and 8.1 times 10-4 l. mol-1 at 2 degrees, indicating that the binding reaction is exothermic. The valences of the pentamer IgM, the 7S IgM subunit produced by reduction with cysteine, and the Fab fragment obtained by cleavage with papain were 10, 2, and 1, respectively. By all criteria available for antibody-like binding such as high specificity, restriction of the binding sites to the Fab part of the molecule and correct stoichiometry this IgM exhibits the fundamental characteristics associated with conventionally induced antibodies.