In order to study the effect of interparticle interactions the magnetic properties of amorphous (Fe1—xNix)100—yBy nanoparticles (0.25 < x < 0.75 and y ≈︂ 0.50) have been investigated by magnetization measurements in a homogeneous dispersion of particles in a PVP polymer and in powder samples. The dispersed samples show the characteristic behaviour of an assembly of independent nanosized particles, characterized by a narrow size distribution, with a sharp maximum of zero field cooled (ZFC) magnetization whereas the field cooled (FC) magnetization increases continuously with decreasing temperature. The magnetization in the superparamagnetic regime is satisfactorily fitted to a superposition of Langevin functions due to the volume distribution. On the other hand, in the powder samples, where strong interparticle interactions are expected, the MZFC shows a wider maximum with a significant increase of the temperature at which it occurs and MFC presents a plateau at low temperature. The magnetization curves for the powder samples can be fitted to a Langevin function if interparticle interactions are taken into account by introducing a mean-field-like term in the argument.