The teratogenic effect of RA was found to be significantly influenced both by genetic background and by the genotype of malformation mutation Lx. The presence of the Lx mutation and BN genetic background strongly increases the teratogenic effect of RA. On the contrary, the SHR genetic background was shown to protect foetuses from RA teratogenic affliction. Recombinant inbred strain BXH2 is endowed with a specific combination of BN and SHR genes, and following RA administration it exhibits the same embryolethal effect as the BN genetic background alone. Without the Lx mutation there was no effect of RA on hind limbs in SHR/SHR or SHR/BN progeny whilst there was a significantly higher occurence of oligodactyly in SHR/BN on forelimbs as compared to SHR/SHR (92.2% vs 11.5%). In +/Lx progeny, forelimbs were significantly more afflicted with oligodactyly in SHR/BN +/Lx in comparison with both SHR/SHR and SHR/BXH2 foetuses, which indicates that BN modifiers responsible for oligodactyly were not passed to the BXH2 strain. On the contrary, hind limbs of SHR/BXH2, +/Lx progeny exhibited the highest affliction (62% of polydactyly and/or oligodactyly). In homozygous Lx/Lx progeny, polydactyly prevailed in forelimbs of SHR/BXH2 following RA administration, whilst in BN/BN progeny oligodactyly was the most frequent affliction. On the hind limbs, the highest reduction of toe number after RA treatment was connected with BN modifiers. The polymorphism of normal morphogenetic factors was shown to be responsible not only for Lx. phenotypic manifestation, but also for the variability in the response to RA teratogenic action.