The recently developed early antigen immunofluorescence (IF) method for the detection of infectious cytomegalovirus (CMV) in clinical specimens has hardly been applied on blood samples. We compared the CMV early antigen detection technique with the conventional cell culture method in 415 different buffy coat samples from 85 different immunocompromised patients. Duplicate coverslips were stained with two different monoclonal antibodies 4-6 days after inoculation. The conventional cultures were examined for typical cytopathic effects (CPE) during 10 weeks. Forty samples from 19 patients were positive by the IF technique, most of them with both monoclonal antibodies. Only 22 of these samples were positive in the conventional cell culture assay, on average after 15.8 days. CMV viraemia was detected exclusively by the IF method in 18 samples, 7 of which were from five patients without any further evidence of an active CMV infection. CMV viraemia was detected exclusively by the CPE method in eight samples, on average after no less than 36.6 days. CMV viraemia was not found in blood samples from 10 patients with laboratory proven active CMV infections and 53 patients without any evidence of an active CMV infection. In our hands the early antigen method for the detection of infectious CMV in blood is nearly as specific (at least 98.1%) and clearly much faster and more sensitive than the conventional cell culture method. The early CMV antigen detection method is therefore a very useful tool for the rapid detection of infectious CMV in blood.