The effect of neonatal gene transfer on antibody formation was determined using a retroviral vector (RV) expressing human factor IX (hFIX). Normal mice from different strains injected intravenously with RV as newborns achieved therapeutic levels of hFIX without antibody production and were tolerant as adults to challenge with hFIX. Neonatal hemophilia B mice that received different amounts of RV achieved stable and dose-related expression of hFIX without anti-hFIX antibody formation. After protein challenge, antibody formation was markedly reduced for animals that expressed hFIX at levels higher than 14 ng/mL (0.3% of normal). However, antibodies developed for animals that received the lowest dose of RV and expressed hFIX at approximately 2 ng/mL before protein challenge. In dogs, neonatal injection of a high dose of RV resulted in 500 ng/mL hFIX in plasma without antibody formation. We conclude that neonatal gene transfer with RV does not induce antibody responses to hFIX in mice or dogs and that mice achieving levels greater than 3 x 10 -10 M hFIX are usually tolerant to protein injection as adults. Low-dose gene therapy or frequent protein injections in the neonatal period might induce tolerance to subsequent injections of protein with a low risk for adverse effects.