Seventy-five accessions of M. orbicularis were screened for resistance to alfalfa mosaic virus (AMV) by application of viruliferous aphids to plants. Thirty of these accessions were then exposed to infection in the field and 22 were graft-inoculated. Three types of resistance to AMV were found among the different accessions. These were 'extreme resistance' in SA 15222, 'systemic hypersensitivity' in SA 2570, SA 9310, SA 10282, SA 12337, and SA 15052, and 'partial resistance', a type that was intermediate between the two, in SA 5073, SA 13905, and SA 17711. M. orbicularis accession SA 10282 (hypersensitive) was crossed reciprocally with SA 4268 (susceptible), and the resulting Fl plants were allowed to self naturally or were backcrossed with SA 4268. F-2 progeny and backcross progeny plants were graft-inoculated with AMV to determine segregation ratios (hypersensitive:non hypersensitive). The ratios from both the crosses and reciprocal crosses and from the backcross populations indicated that the hypersensitive response to AMV may be controlled by a single dominant gene for hypersensitivity for which we tentatively propose the name Nam-1. M. orbicularis accessions provide a valuable source of AMV resistance for use in breeding virus-resistant cultivars of a range of useful annual medic species.