The measles virus (MV) hemagglutinin (H) gene nucleotide sequences of the LEC-WI strain and 11 branched sequential neutralization escape variants of the strain derived by selection with five monoclonal antibodies (Mabs) were determined by direct analysis of amplified polymerase chain reaction products. The parental LEC-WI strain isolated from a patient with subacute sclerosing panencephalitis exhibited H gene sequence characteristics similar to other persistent virus strains derived from brain materials. Mostly single-point H gene mutations, coding for single amino acid substitutions in the H protein, were found to provide explanations for the resistance to the individual Mabs. Resistance to Mabs 16-CD11 and 1-41 resulted from changes of Gly-491 to Asp (or Val) and Phe-552 to Val, respectively. One variant (B89) selected by Mab 16-CD11 had a mutation introduced by a single nucleotide deletion and subsequent nucleotide insertion, which caused a shift in the open reading frame. The epitope of Mab I-29 was assigned to Ser-313 or Gly-314, which were changed to Leu and Arg, respectively. The variants subjected to the Mab I-44 selection exhibited change of Ser-189 to Pro. Radioimmunoprecipitation assay and endoglycosidase H (Endo H) treatment revealed that this change destroyed a potential N-linked glycosylation site, indicating that the carbohydrate chain participates in formation of the epitope or indirectly influences its properties. Resistance to Mab 16-DE6 involved three specific amino acid changes in three different places, Gly-211 to Ser, Gly-388 to Asp, and Ser-532 to Phe or Arg-533 to Gly, reflecting the occurrence of a conformational epitope. In conclusion, this study identifies the precise positions of several critical sites on the MV H protein which react with neutralizing antibodies.