Five different ELISA procedures using alkaline phosphatase conjugates were compared for their sensitivity in detecting binding activity in nine monoclonal antibodies raised against tobacco mosaic virus (TMV). The two procedures in which antigen was used to coat the microtiter plates were found to be unreliable for testing the activity of monoclonal antibodies. The two most sensitive procedures utilized a multilayered sandwich of which one component imperatively had to be an avian antibody devoid of serological cross-reactivity with mammalian globulins. The avian antibody was obtained from the egg yolk of immunized chickens. In the two most sensitive assays, four out of nine monoclonal antibodies raised against TMV were also reactive with the dissociated viral protein subunits.