Twelve clones of murine hybridoma cells secreting antibody specific for respiratory syncytial (RS) virus were classified into four groups on the basis of their pattern of staining of unfixed RS virus-infected HEp-2 cells in an indirect immunofluorescence test. Three of the groups reacted with virus antigens present on the membrane of the cells, whilst the fourth group failed to stain most live cells, suggesting specificity for an antigen expressed internally. Representative monoclonals from the membrane antigen staining groups immunoprecipitated the 86K glycoprotein (G), 50K plus 19K glycoprotein (F1,2) and a 23K non-glycosylated protein (VP23). A representative monoclonal from the fourth group that appeared to stain an internally expressed protein immunoprecipitated the virion 34K phospho-protein (P). All four monoclonals stained acetone-fixed tissue culture cells infected with either the Long strain of RS virus or with strains isolated in Newcastle during the 1965, 1972, and 1983 winter epidemics. The anti-fusion protein antibody stained acetone-fixed cells from all of 26 nasopharyngeal secretions from infants with RS virus infection. The anti-G glycoprotein antibody and the anti-VP23 antibody stained cells from secretions poorly or not at all, whilst the anti-P protein antibody stained cells in half the secretions tested but reacted with only a small proportion of cells in comparison with the anti-F or polyclonal antibodies. A pool of all four monoclonals produced more intense staining than the anti-F monoclonal alone and gave a more clearly defined staining reaction than the polyclonal antiserum used for routine diagnosis in over half the secretions. These results indicate that monoclonal antibodies will be of value in the diagnosis of RS virus by indirect immunofluorescence if care is taken in the selection of a suitable pool.