Lupinus mutabilis ( pearl lupin) P26956 remained uninfected after repeated sap inoculations with Cucumber mosaic virus (CMV) isolates LW, SN-1, CP, LE, SL and BA. In contrast, plants of L. mutabilis P26961 were readily infected by isolates LW, SN-1 and CP. Isolates LE, SL and BA infected most or all plants of P26961 only after repeated inoculation. When scions of L. angustifolius (narrow-leafed lupin) infected with isolate LW were top-grafted onto plants of L. mutabilis P26956, necrosis developed in the shoots produced just below the graft union whereas other axillary shoots grew vigorously with no virus being detected. The responses to sap inoculation with diverse isolates and to graft inoculation suggest that L. mutabilis P26956 has 'extreme resistance' to CMV.