Cell-cultures of cytomegalovirus (CMV) were fixed after 24 hours' incubation and examined by a monoclonal antibody based immunofluorescence method for the detection of CMV-specific early antigens. 385 urine, saliva, or blood samples from 63 immunocompromised patients were inoculated onto cell-cultures. Comparison with the results of conventional cell-cultures in patients who remained uninfected showed that the new technique had a specificity of 100%. The sensitivity was 80%. This immunofluorescence method gave positive results 27h after inoculation of the specimens instead of the mean of 17 X 5 days with the conventional method based on detection of cytopathic effect. 3 saliva samples, from patients who had previously excreted CMV, reacted in the immunofluorescence method but CMV, reacted in the cell-cultures-perhaps because the assay identified defective, interfering particles in these samples. The monoclonal antibodies were also used successfully in another immunofluorescence system to diagnose cytomegalovirus pneumonitis in 3 patients by testing material obtained by bronchoalveolar lavage.