The fusion (F) glycoprotein of respiratory syncytial virus (RSV) induces neutralizing antibodies and antibodies that inhibit fusion of infected cells (FI antibody). It was previously shown that infants and children immunized with Formalin-inactivated RSV 20 years ago developed antibodies that bound to the F glycoprotein but were deficient in neutralizing activity. A reexamination of these sera indicated that they were also deficient in FI activity. Thus, Formalin-inactivated RSV vaccine stimulated an unbalanced immune response in which an unusually large proportion of the induced antibodies were directed against nonprotective epitopes rather than against the epitopes that induce functional antibodies, i.e., neutralizing and FI antibodies. This deficiency in stimulation of functional antibodies probably decreased the protective efficacy of the vaccine and could have contributed to potentiation of disease in the vaccines during subsequent RSV infection.