Five monoclonal antibody (MAb) neutralization escape mutants of respiratory syncytial virus (RSV) were produced by growing the Long strain RSV (group A virus) in the presence of a neutralizing, group cross-reactive MAb specific for the attachment protein (G). Four viruses (RSV-2, -6, -14 and -15) had amino acid replacements clustered within a highly conserved centrally located 13 amino acid region (position 164-176). Reactivity with group A-specific MAbs and with polyclonal anti-G serum was maintained and growth kinetics were unaffected. An additional virus (RSV-3) had four amino acid substitutions in the cytoplasmic tail and transmembrane region of G, and had restricted growth and formed small syncytia. Immunofluorescent and Western blot analysis indicated that G protein was not membrane associated and had reduced incorporation into the virion, thereby escaping neutralization by L9 and polyclonal anti-G serum. The predominant form of G produced by RSV-3 was found in infected cell supernatants, consistent with the size of secreted G.