The effect of passively transferred RSV immune serum on the antibody response to a single dose of purified RSV fusion (F) and large (G) glycoproteins was studied in cotton rats. Passively transferred antibody that achieved serum antibody levels similar to those seen in newborn human infants resulted in a seven- to eightfold suppression of the neutralizing antibody response of cotton rats to low doses of purified F and G glycoproteins (0.2-1.7 micrograms) and a twofold suppression to higher doses of these antigens (5-15 micrograms). This suppression of the antibody response was accompanied by a reduction in the protective efficacy of the F and G purified glycoprotein vaccine. These results suggest that parenteral immunization with RSV antigens could be less immunogenic in seropositive human infants, but that this suppressive effect might be partially overcome with increased antigen dose.