Infants and young children undergoing primary infection with respiratory syncytial virus (RSV) develop moderate to high levels of antibodies to the F and G surface glycoproteins of this virus, but only a very small proportion of these antibodies exhibit neutralizing activity. The development of antibodies during primary RSV infection which are predominantly devoid of an important function required for immunity, i.e., virus neutralization, may contribute to the severity of initial infection and may also be responsible, in part, for the ineffectiveness of resistance to frequent subsequent reinfection by RSV, with associated respiratory tract disease, during early childhood.