We have prepared and characterized monoclonal antibodies against five major structural proteins, i.e. the HA, P, NP, F and M proteins, of measles virus. At least three non-overlapping antigenic sites were delineated on the HA protein, three on the P, four on the NP, four on the F and five on the M proteins by competitive binding assays. Antigenic sites on the HA and F proteins roughly represented functional domains defined by serological tests. The reactivity of monoclonal antibodies with various measles virus strains including those from subacute sclerosing panencephalitis (SSPE) and other members of the morbillivirus family was studied by immunofluorescence. A monoclonal antibody or set of monoclonal antibodies to each of the antigenic sites showed a characteristic pattern of cross-reactivity with heterologous strains. The HA and NP proteins were antigenically the most variable, followed by the F and M proteins, while the P protein was relatively stable. None of the 14 anti-M monoclonal antibodies reacted with non-virus-producing SSPE cells, strongly suggesting the absence of M protein in these cells.