In an organism with a life-cycle modelled on that of Plasmodium two separate resistance
genes are assumed, each protecting against one of two unrelated drugs. The model was
used to compare the rates of build-up of resistance in a population where the two drugs are
used either as a mixture or in sequence. The model suggests that the use of a mixture would
be advantageous if:(i) both resistances are initially rare;(ii) there is recombination between
the genes;(iii) a large proportion of the parasite population is unexposed to the drugs.