Cyclic deformations of annealed pure polycrystalline alpha-iron with and without further mutually immiscible silver-ion implantation (90 keV, 6 x 10(16) ions cm(-2)) were studied in a plastic strain-controlled tension-compression fatigue test (triangular loading waveform, frequency 0.02-0.3 Hz, and plastic strain range 3 x 10(-3)-1.2 x 10(-2)). The obtained plastic strain-life (Delta epsilon(p)-N-f) curves showed that the iron specimens could survive for a greater number of cycles before failure when implanted. Comparison of the cyclic stress-strain curves suggested that the implanted specimens had maintained a relatively more stable microstructural change than those unimplanted ones which had undergone a violent cyclic hardening during cyclic deformation. This is proposed to be a strong indication that the fatigue ductility has been improved and the cross slip of screw dislocations, which leads to the evolution of the persistent slip bands for fatigue damage, was hindered to some extent after ion implantation.