A new one-step method was developed to detect IgG antibodies to different virus antigens. Human serum specimens were mixed with different enzyme-labelled antigen (ELA) preparations in microtitre plates coated with rheumatoid factor (RF). During the incubation in the wells of microtitre plates immune complexes form which are specifically bound to the RF on the solid phase. The test detects antibody to cytomegalovirus (CMV) and West-Nile virus with a high sensitivity and specificity.