By means of indirect immunofluorescence microscopy we have shown that 11 out of 18 (61%) IgM M-components from patient sera had affinity for various tissue structures. The great variation in affinity patterns, and the high titres support the antigen-antibody mode of interaction. The high incidence of activity against smooth muscle (4/18) shown in this study, and against IgG, and red blood cell antigens shown by others indicates that the onset of IgM M-component production is initiated in situations where a polyclonal IgM-antibody response is the rule. Beside specific antibody activities, the M-components tested showed affinity for gastric epithelium and hepatocytes. The titres correlated roughly to the serum concentrations of IgM, and the patterns corresponded to the localization of secretory component as shown by others indicating a non-antigen--antibody reaction between the IgM M-components and secretory component.