Neutralizing polyclonal antibodies to RSV (RSVIG) have been shown to be effective prophylactic agents when administered intravenously in high risk infants. Newer prophylactic and/or therapeutic agents may include monoclonal antibodies (Mab's) which should have a higher specific activity, allowing enhanced patient delivery. Salutory properties of an effective Mab against RSV would include potent high affinity neutralizing activity, broad reactivity with clinical isolates, and efficacy in a relevant animal model. To this end, we have developed MEDI-493, a humanized Mab (IgG1, Kappa) derived from the murine Mab 1129 which recognizes a conserved neutralizing epitope on the F glycoprotein of RSV (Beeler and Coelingh). In this study, the binding and functional properties of MEDI-493 were determined using ELISA and biosensor assays for affinity, three in vitro neutralization assays and an in vivo cotton rat model of protection. The affinity of MEDI-493 was found to be 1.4 nM, equal to or slightly better than an isotype matched chimeric derivative of Mab 1129. In microneutralization and fusion inhibition assays, MEDI-493 was 20-40 fold more potent than the polyclonal preparation. Fifty percent neutralization of RSV A (Long) and B (18537) laboratory strains was obtained at a Mab concentration of approximately 2 ug/ml in a plaque reduction assay. Broad neutralizing activity with a panel of over 50 clinical isolates of the RSV A and B subtypes was shown also. Pretreatment of cotton rats with MEDI-493 demonstrated a 99% reduction of lung RSVtiters with 2.5 to 5 mg/kg of the Mab as compared with 125 to 250 mg/kg of polyclonal RSVIG. This corresponded to a serum level of MEDI-493 at the time of challenge of approximately 25-30 ug/ml. Further, MEDI-493 did not induce increased RSV infection or pathology in either a primary or a secondary challenge. Thus, MEDI-493 was shown to be very potent when compared to RSVIG preparations, in each of these in vitro and in vivo studies. These results provide the rationale for clinical studies to evaluate this humanized Mab in children at risk for RSV.