The hindering of crystallization upon the addition to metal alloys of perturbing nanoparticles is theoretically analyzed. It is shown that the effect of the particles is to add a surface energy term to the Gibbs energy of formation of the crystalline nucleus. This change in the Gibbs free energy slightly modifies the effective fusion temperature, but significantly changes the nucleation frequency, Besides this surface interaction, the particles introduce an additional effect, the reduction of the volume fraction occupied by the crystallizing phase. The effect of the perturbing particles is characterized by two independent parameters, the volume fraction of particles and another parameter that depends on the particle size, the interfacial energies between the particles and supercooled liquid, and the heat of fusion of the alloy.