For assessment of spleen necrosis virus (SNV) as a gene transfer vector in pig and manse, the infectivity of SNV to zona-free preimplantation stage porcine and mouse embryos following co-culture with cells producing SNV was examined. Also, the function of SNV long terminal repeat (LTR) fused with chloramphenicol acetyl transferase (CAT) gene in transgenic mice was evaluated. A total of 15 pig fetuses at 6 weeks gestation derived from co-culture of zona-free 4-cell stage porcine embryos with cells producing SNV were analysed for the integration of SNV provirus sequences in their tissues. Analysis using PCR and DNA blots indicated absence of SNV sequences in all fetuses samples. Also, zona-free 2- and 4-cell stage mouse embryos were co-cultured with cells producing SNV. Thirty one pups derived from co-culture experiment and G₁ offspring of live G_0 mice were analysed for the presence of SNV provirus sequences in their body using DNA blots. All mice tested were negative for the SNV provirus sequence. Transgenic mice containing SNV LTR fused with CAT gene were produced by microinjection to investigate the regulatory function of the SNV LTR in mice. The expression of CAT gene was below the level of detection in assayed tissue of 5 tzansgenic lines that retain several copies of SNV LTR-CAT sequences. Overall infectivity of SNV to pig and mouse embryos did not appear high enough to be useful for gene transfer in pig and mouse. SNV LTR has not shown to be functional in mice.