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In the paper we present the results of the Monte Carlo simulations of time-of-flight transient currents in hopping systems containing various concentrations of transport-active centres placed in an inert matrix, and doped with various concentrations of additional transport-active centres, characterised by a different ionisation potential than the host transport centres. Such hopping centre distribution can be found in molecularly doped organic crystals. The carriers are allowed to jump between localised states of two kinds: the host transport sites of concentration c0, and the additional doping sites of concentration cd. We deal only with the dependence of the carrier mobility on doping level, and on the difference of ionisation potentials between the host and dopant centres, at fixed external field and temperature. The results of simulations performed for various dopings, and various hopping sites distributions in energy, are discussed. The carrier jump statistics is discussed in detail.