Intravenous injection of New Zealand White rabbits with type III pneumococcal polysaccharide vaccine conjugated with the cardiac glycoside digoxin resulted in the production of both antidigoxin and anti-type III pneumococcal polysaccharide antibodies. Among antisera of 12 rabbits examined during their peak antibody production periods, 1 to 20 mg (mean, 5.4 mg) of antidigoxin antibody could be recovered from 1 ml of serum. Antisera from five of these 12 rabbits contained antidigoxin antibodies of restricted heterogeneity as demonstrated by urea-polyacrylamide disc gel electrophoresis of fully reduced and alkylated antibodies. From the antisera of four of these five rabbits, electrophoretically homogeneous antibodies (1 to 5 mg/ml antiserum) could be isolated by affinity chromatography on ouabainamine-Sepharose columns. The structural homogeneity of two of these antidigoxin antibodies was confirmed by amino acid sequence analysis of purified light chains through the first hypervariable region. These data suggest that the conjugation of small molecules to bacterial polysaccharide vaccines may provide a general method for synthesis of immunogens that can regularly elicit antihapten antibodies of restricted heterogeneity.