The magnetization of 0.3 and 140 Hz and Mössbauer spectra on Fe57 have been measured in the temperature range between 4.2 and 300 K for polycrystalline (Fe65Ni35)1-x(Fe84 Mn16)x (0x0.26) alloys. Two characteristic temperatures, Tgac and Tiac (Tgac>Tiac) were observed in ac-T curves for x=0.130.24 alloys. The temperature Tiac coincides with the temperature below which an average hyperfine field increases steeply. These results reveal that the alloys undergo a transition from a ferromagnetic to a spin-glass state at Tiac. The reentrant spin-glass phenomena for this alloy system can be interpreted by the picture of the existence of antiferromagnetic clusters caused by a statistical fluctuation of alloy composition in the ferromagnetic matrix.