Technical Abstract: Porcine reproductive and respiratory syndrome virus (PRRSV) inhibits synthesis of type I interferons (IFNs) in infected pigs and in cultured cells. Here we report that one PRRSV mutant A2MC2 induces type I IFNs in cultured cells and has no effect on IFN downstream signaling. The mutant isolate was propagated in a monkey kidney-derived cell line, MARC-145. Pre-treatment of Vero cells with culture supernatant from A2MC2-infected MARC-145 cells led to inhibition of LaSota Newcastle disease virus (NDV), an IFN-sensitive virus. The A2MC2 strain was confirmed to be Type 2 PRRSV by Western blotting with pig convalescent antiserum and by an inhibition assay with a PRRSV-specific antisense morpholino oligomer. A2MC2 replication in MARC-145 cells resulted in elevation of the transcripts of IFN-stimulated genes, ISG15 and ISG56, and the proteins of the signal transducer and activator of transcription (STAT) 2 and p56. A2MC2 infection of primary porcine pulmonary alveolar macrophages (PAMs) also led to elevation of STAT2 and p56, but little cytopathic effect. Furthermore, IFN signaling in either A2MC2-infected MARC-145 or PAM cells was not affected as no detectable decrease in the expression of the IFN-inducible genes was discovered in the cells that were treated with an external IFN-a, while other PRRSV strains, such as VR-2385, inhibit the expression of these genes. Sequencing analysis indicates that A2MC2 is closely related to VR-2332 and Ingelvac PRRS MLV. The identification of this IFN-inducing PRRSV isolate might be beneficial for vaccine development to induce protective immunity against PRRS.