Mice were immunized with various antigens in complete Freund's adjuvant following various injection schedules. Hybridomas were produced from the spleens of these immunized mice and examined for production of antibodies directed against the antigen injected and against a panel of self (tubulin, actin, myosin, DNA) and non-self antigens (myoglobin, spectrin, peroxidase, trinitrobenzene). Two to five percent of the hybrids were found to secrete polyspecific antibodies able to react with two or more antigens of the panel. Several of these hybrids were subcloned and expanded into ascites. The monoclonal immunoglobulins they secreted were isolated and shown to be IgM (kappa) and to possess the polyspecific antibody function. Several hybrids were also found to secrete antibodies reacting with the immunizing antigen as well as one or more antigens of the panel. The antibody secreted by one subclone which reacts with both the immunizing antigen, prolactin and one of the panel antigens, TNP, has been isolated using a DNP-immunoadsorbent. The isolated antibody was found to be a monoclonal IgM (kappa) immunoglobulin and to react both with prolactin and TNP. The hypothesis is advanced that cells carrying polyspecific natural antibodies as receptors after a given antigenic stimulation proliferate into cells producing highly specific antibodies for epitopes of that given antigen; the cells with polyspecific receptors will be continuously replaced by new cells probably on bone-marrow origin.