Maternal Abs generated as a result of prior exposure to infectious agents such as the malaria parasite are transferred from the mother through the placenta to the fetus. Numerous studies have attributed the resistance to malaria infection observed in neonates and infants up to 6 mo of age to the presence of maternally derived Abs. However, recent studies have produced conflicting results suggesting that alternative protective mechanisms may be responsible. Although the presence of maternally derived Abs in the infant is not disputed, their exact role in the infant is unknown. Even less clear is the effect that maternally derived Abs, if generated in response to vaccination, may have on the infant's ability to respond to malaria infection. Studies on mouse pups were performed to determine the role of the 19-kDa region of merozoite surface protein 1 (MSP1(19)) and Plasmodium yoelii-specific Abs in neonatal malaria infection and to examine their effect on the development of a specific immune response in the pup. It was shown that P. yoelii- and MSP1(19)-specific Abs transferred to the pup from the mother act to suppress the growth of the parasite in the pup. However, the maternally derived Abs interfered with the development of the pups' own Ab response to the parasite by altering the fine specificity of the response. These results suggest that immunizing women of child-bearing age with a malaria vaccine candidate such as MSP1(19) would not prevent the infant from producing Abs in response to malaria infection, but it may affect the region of the Ag to which it responds.