In vitro fertilizability and subsequent developmental competence of inbred RFM/Ms mice, which have high incidence of myeloid leukemia by radiation, was studied. When response of RFM/Ms females to various amount of eCG (1.25-10.0 i.u.) was examined, superovulation (>20 ovulated ova) occurred in animals injected with ≥5.0 i.u. eCG. The kinetics of in vitro fertilization and embryo development were examined and compared among RFM/Ms, C57BL/6J zygotes and their hybrid zygotes. The fertilization kinetics were significantly slower in RFM/Ms zygotes than in C57BL/6J zygotes. Crossbreeding experiments between C57BL/6J and RFM/Ms showed that the speed and incidence of fertilization were affected by paternal and maternal factors, respectively. The in vitro development to the blastocyst stage of RFM/Ms zygotes was significantly slower than that of C57BL/6J zygotes and their hybrid zygotes at 96 h post-insemination (PI), but not at 120 h PI. The nuclear number of RFM/Ms embryos was significantly lower than that of C57BL/6J and their hybrid embryos throughout the culture period (72, 96 and 120 h PI). Conversely, hatching at 120 h PI in C57BL/6J embryos was much slower than that of the other groups of embryos. This study showed that gametes and zygotes of RFM/Ms strain can be fertilized and developed well in vitro and that maternal and paternal factors and their interaction have complicated effects on fertilization and developmental kinetics in vitro.