Mx genes are inducible by type I interferon and involved in antiviral defenses in vertebrates. In the present investigation, the protocol for obtaining Mx gene expression was standardized by Poly I:C injection in Labeo rohita. A 786 bp cDNA was obtained on day 3 after poly I:C injection. PCR amplification of this cDNA using Mx specific primers also yielded 786 bp product. However, PBS injected fishes did not show any Mx expression.