The transcription of Mx mRNA after poly I:C induction and sole aquabirnavirus infection has been analysed in SAF-1 and TV-1 cells (derived from gilt-head seabream and turbot, respectively). Both cell lines were stimulated with 10 microg ml(-1) poly I:C and Mx mRNA was analysed by a specific RT-PCR at several times post-induction. The results showed a high level of Mx expression from 12 to 120 h after induction in SAF-1 cells, whereas in TV-1 cells Mx mRNA was only detected at 12 and 24h. The treatment with different concentrations of poly I:C showed that TV-1 cells are less sensitive to this inductor than the SAF-1 cell line. The antiviral activity derived from poly I:C induction has been clearly demonstrated against sole aquabirnavirus on both cell lines. The inoculation of sole aquabirnavirus resulted in the Mx mRNA transcription at 48, 72, and 96 h post-infection (p.i.) in SAF-1 cells. On the contrary, inoculated TV-1 cells only showed a faint Mx mRNA band at 24 and 48 h p.i. This study has established different patterns of Mx expression in both cells under study as a consequence of the poly I:C induction and sole aquabirnavirus infection, and it shows that gilt-head seabream and turbot Mx inhibit sole aquabirnavirus replication.