||Our previous studies have hypothesised that a complementary epistasis between a QTL located on chromosome 12 and a QTL located on chromosome 7 was one of the major genetic factors controlling partial resistance to Rice yellow mottle virus (RYMV). We report research undertaken to verify this hypothesis and to introgress the resistant allele of these two QTLs from an upland resistant japonica variety, Azucena. into a lowland susceptible indic-a variety IR64. Three cycles of molecular marker-assisted back cross breeding were performed using RFLP and microsatellite markers. Resis- tance to RYMV was evaluated in F2 and F3 offspring of the BC( and BC2 generations. Marker-assisted intro- gression (MAl) was very efficient: in the selected BCJ progeny the proportion of the recipient genome was close to 95% for the ten non-carrier chromosomes, and the length of the donor chromosome segment surrounding the two QTLs was less than 20 cM. The relevancy of the complementary epistasis genetic model proposed previously was confirmed experimentally: in BC( and BC2 generations only F3 lines having the allele of the resistant parent on QTL12 and QTL7 show partial resistance to RYMV. Comparison of our experimental pro- cess of MAI with the recommendations of analytic and simulation studies pointed out the methodological flexibility of MAI. Our results also confirmed the widely admitted, but rarely verified, assumption that QTL-alleles detected in segregating populations could be treated as units of Mendelian inheritance and that the incorporation of these alleles into elite lines would result in an enhanced performance. The next step will be the design of tools for the routine use of molecular markers in breeding for partial resistance to RYMV and the development of material for the analysis of resistance mechanisms and the structure of a virus resistance gene in rice.