SUMMARYRabbit antisera produced against two comoviruses (cowpea mosaic virus and cowpea severe mosaic virus) were used in plate-trapped ELISA and liquid phase competition ELISA. In the latter, the top component competed against bound unfractionated virus more effectively than did nucleoprotein components. One murine monoclonal antibody elicited to cowpea mosaic virus and three monoclonal antibodies to cowpea severe mosaic virus exhibited differential binding to top component, relative to either middle or bottom components in plate-trapped or antibody-trapped ELISAs. Differential binding to centrifugal components of virus by two of these monoclonal antibodies was maintained in liquid phase competition assays, These data suggest that the encapsidation of RNA alters the configuration of the virion surface in the vicinity of the antibody epitopes. Ribonuclease digestion of intact or denatured virus did not affect the binding of monoclonal antibodies.Like other comoviruses, cowpea mosaic virus (CPMV) and cowpea severe mosaic virus (CPSMV) are resolved into three major fractions by density gradient centrifugation (Bruening, 1969). The slowest sedimenting top component consists of empty capsids whereas the middle and bottom components are nucleocapsids containing one molecule of either RNA 2 (Mr 1"37 x 106) or RNA 1 (Mr 2.02 x 106), respectively. PAGE does not show differences between the protein composition of the three components (Geelen et al., 1972;Wu & Bruening, 1971) and rabbit antisera prepared against unfractionated virus reacts with all components in Ouchterlony double-diffusion assays (Bruening & Agrawal, 1967). This paper reports that liquid phase competition assays utilizing rabbit antisera reveal a difference between the binding of antibodies to empty capsids and to nucleoprotein virions. Antigenic differences between top component and middle or bottom components were confirmed by assays with monoclonal antibodies.CPSMV-DG and CPMV-SB isolates were originally obtained from G. Bruening (Department of Plant Pathology, University of California, Davis, Ca., U.S.A.), propagated in cowpeas (Vigna unguiculata Walp. cv. California Black Eye) and isolated as previously described (Bruening, 1969). Approximately 5 mg virus was suspended in 0-5 ml gradient buffer (50 mM-KH2PO, pH 7.0, 2 mM-disodium EDTA) and layered onto 11 ml of 39 ~ (w/w) caesium chloride in gradient buffer. Gradients were centrifuged for 36 h at 36000 r.p.m, and 4 °C in an SW 40Ti rotor (Beckman) and fractionated by side-puncture. Fractions were diluted at least 10-fold in gradient buffer and pelleted by centrifugation at 45000r.p.m. for 2h. The pellets were resuspended in gradient buffer. Concentrations of virus components were calculated using A260 at 1 mg/ml of 6.2, 10-0 and 8.1 for middle component, bottom component and unfractionated virus and A2so at 1 mg/ml of 1.28 for top component (Geelen et al., 1972).The preparation of rabbit antisera and of the panel of monoclonal antibodies produced by hybridomas have been described (Kalmar & Eastwell,...