fjThe effect of microstructural inhomogeneities on the rotating-bending-fatigue response of a pre-alloyed (FL-4405) and two hybrid (FLC2-4405 and FLN2-4405) steels was evaluated. Different microstructures at a nominal density of 7.4 g/cm(3) were developed by conventional sintering, high-temperature sintering, quenching and tempering, and sinter hardening followed by tempering, In previous studies on these steels, tensile and impact properties, hardenability, fatigue-crack-growth rates, pore characteristics and residual-stress distributions were quantified. For each steel, the highest fatigue limit but the lowest fatigue ratio is obtained in the quenched + tempered condition. Sinter hardening of the steels containing copper and nickel increases the fatigue limit relative to the as-sintered condition. High-temperature sintering reduces the fatigue limit relative to conventional sintering. The fatigue ratio is a function of microstructure and is lowest in the three steels in the quenched + tempered condition. The inferior fatigue behavior of the copper-containing steel is attributed to the large pores resulting from the coarse copper powder.