The exposure of a mixture of 1 mM HAuCl4 and 1 mM AgNO3 solutions to different amounts of fungal biomass (Fusarium oxysporum) results in the formation of highly stable Au-Ag alloy nanoparticles with dimensions of 8-14 nm depending on metal molar fraction. The amount of cofactor NADH released by the F. oxysporum fungus plays an important role in controlling the composition of the alloy nanoparticles