(CBA/N X BALB/c male)F1 mice bear on X-linked defect making them totally unresponsive to T-independent (TI), TI-2 antigens such as type III pneumococcal polysaccharide (SSS-III). We found that somatic cell hybrids between CB nonresponder spleen cells and NS1 plasmacytoma cells secreted antibody specific for SSS-III. The solid-phase binding of such antibody was completely inhibited by the addition of free antigen (SSS-III) and the amount of antibody detected in culture fluids ranged from 10 ng/ml to 10 micrograms/ml. Eight hybridoma clones were identified; all make antibody of the IgM class. These results indicate that the X-linked defect does not result in a deletion of a B-cell subset which responds to TI-2 antigens.