Respiratory syncytial virus (RSV), strain Long, was purified through 20-60% sucrose gradient. The virions from different sucrose density zones were tested by ELISA for reactivity with monoclonal antibodies (MAB) to F- (MAB 9C5) and N- (MAB 8B10) proteins of RSV. Comparative study of the same patterns of RSV by electron microscopy after negative staining showed a close relationship between the virion morphology and MAB binding in ELISA. MAB 9C5 were highly reactive with the surface domains of both mature RSV virions and "empty" virion envelopes without formed inner nucleocapsid structures. MAB 8B10 reacted well only with mature virions with completely assembled nucleocapsids. These MAB failed to reorganize the N-protein epitope of immature and destroyed virions, which indicated a conformation dependence of the 8B10 binding site. For practical purposes, MAB tests can be used to determine the RSV patterns, which can be used in ELISA for serologic diagnosis of RSV infection. Testing with these MAB demonstrate the stability of RSV to extreme exposures (lyophilization, storage, heating), which is important for creation of sensitive ELISA test systems and their standardization.