In the present study, the role of sodium fluoride on antibody formation in rabbits is assessed. Sixteen female albino rabbits were divided into four groups and were treated as follows: Group I: Animals immunized with transferrin and used as controls for group II; Group II: Animals immunized with transferrin and administered orally NaF (10 mg/kg body weight) daily for 9 months; Group III: Animals immunized with transferrin and served as controls for group IV; and Group IV: Animals administered with NaF at a dose of 10 mg/kg body weight daily for 9 months and then immunized with transferrin and fluoride treatment continued for another 9 months. Rabbits were bled just prior to immunization and subsequent bleeding was at weekly intervals. Circulating anti-transferrin titers were estimated. DNA and protein synthesis was determined by incorporating [3H]thymidine and [14C]leucine, respectively. The present report demonstrates that sodium fluoride inhibits antibody formation in rabbits. There is a threshold level of fluoride (0.78 ppm) in circulation which is responsible for the inhibitory effect on antibody formation. Inhibition of DNA and protein synthesis by NaF is also demonstrated. It is concluded that fluoride inhibits the antibody formation by decreasing the proliferation of lymphocytes and by inhibiting the protein synthetic ability of immunocytes.