58-percent Ni-Fe alloys have been investigated. It was found that the magnetic properties of the alloys depend on the whole chemical composition: matrix + alloying + impurity. The amount of impurities may be influenced deliberately by a proper selection of the atmosphere, temperature and time of the heat treatment. The effect of magnetic annealing was also investigated. Approximately linear relationship was found between the induced anisotropy constant and the suitably weighted average of oxygen and carbon contents. Our measurements suggest that the total amount of impurities in interstitial solid solution has an influence on value of the induced anisotropy constant.