Summary.-Immune complexes could be isolated from sera of 7 patients with oat-cell carcinoma of the lung, but not from 5 normal controls, using zonal ultracentrifugation. After ultracentrifugation, fractions containing macromolecular IgG were absorbed on a protein A-sepharose column and the immune complexes were eluted and dissociated by glycin-HCl buffer at pH 3-5. The eluates were tested for the presence of tumour-associated proteins as carcinoembryonic antigen (CEA), nonspecific crossreacting antigen (NCA), a2 pregnancy associated antigen (a2PAG) and isoferritin. Whereas none of these tumour-associated antigens could be demonstrated, an ACTH-like activity was detected in the immune-complex fractions of 4 patients with oat-cell carcinoma, by radioimmuno-and bioassay. Polyacrylamide electrophoresis of an immune-complex fraction from a patient with Cushing syndrome showed ACTH-like activities, with mol. wt of 110,000, 75,000, 30,000 and <20,000 (all glycoproteins) indicating the presence of different subfractions of big ACTH.