Twenty-five monoclonal antibodies (Mab) to respiratory syncytial virus (RSV) and two to hepatitis B virus were inoculated intravenously into mice. Twenty-four hours later the mice were challenged intranasally with RSV. Eleven of 14 Mab against fusion protein and four out of six Mab against a larger glycoprotein (GP84) significantly reduced the titre of RSV in the lungs when mice were killed 5 days later. Five Mab against three other RSV proteins and two Mab against hepatitis B virus had no significant effect on RSV infection. These results indicated that serum IgG against one epitope on the fusion protein and another on the larger glycoprotein (GP84) will completely protect mice against challenge. These epitopes are primary candidates for an RSV vaccine produced by techniques of gene cloning and peptide synthesis.