Human metapneumovirus (hMPV) is a recently described member of the Paramyxoviridae family/Pneumovirinae subfamily and shares many common features with respiratory syncytial virus (RSV), another member of the same subfamily. hMPV causes respiratory tract illnesses that, similar to human RSV, occur predominantly during the winter months and have symptoms that range from mild to severe cough, bronchiolitis, and pneumonia. Like RSV, the hMPV virus can be subdivided into two genetic subgroups, A and B. With RSV, a single monoclonal antibody directed at the fusion (F) protein can prevent severe lower respiratory tract RSV infection. Because of the high level of sequence conservation of the F protein across all the hMPV subgroups, this protein is likely to be the preferred antigenic target for the generation of cross-subgroup neutralizing antibodies. Here we describe the generation of a panel of neutralizing monoclonal antibodies that bind to the hMPV F protein. A subset of these antibodies has the ability to neutralize prototypic strains of both the A and B hMPV subgroups in vitro. Two of these antibodies exhibited high-affinity binding to the F protein and were shown to protect hamsters against infection with hMPV. The data suggest that a monoclonal antibody could be used prophylactically to prevent lower respiratory tract disease caused by hMPV.Respiratory viruses account for a large proportion of upper and lower respiratory tract illness in humans. In the past few decades, many etiological agents of respiratory tract illness have been identified. Of these, respiratory syncytial virus (RSV) is the single most important cause of respiratory infections during infancy and early childhood (29). However, only 60% of clinically attended respiratory infections of infants and children are of a known etiology (21). Recently, van den Hoogen et al. (26) discovered and described human metapneumovirus (hMPV) and revealed that it may account for a portion of these previously unclassified infections. Prospective and retrospective studies suggest that hMPV infections account for between 3% and 15% of respiratory tract infections (5,6,8