Extracts of spleen cells obtained from a rabbit hyperimmunized with Type III pneumococcal vaccine and transformed with simian virus 40, were subjected to agarose gel electrophoresis in the presence of an [(125)I]S3 polysaccharide-protein conjugate. Binding of radioactivity in the gamma globulin region was observed. Extracts and media obtained from labeled cell cultures contained a protein that bound to an S3, but not to an S8 or inactivated S3 immunoadsorbent. After elution, the bound protein showed a single band on isoelectric focusing which corresponded in isoelectric point to one of the serum anti-S3 antibodies of the donor rabbit. These observations strongly suggest that a normal committed lymphoid cell may be brought into continuous culture by virus transformation and yet retain its ability to synthesize specific antibody.