A novel form of Rugged dsRNA with a unique composition and physical characteristics was identified with high specificity of binding to TLR3, which conveys an important range of therapeutic opportunities. Unlike the previous known antiviral Ampligen® (poly I, poly C12,U) the new and improved form (poly I, poly C30,U) has a reduced tendency to form branched dsRNA which results in increased bioactivity due to an increased ability to bind TLR3 receptor. Pharmaceutical formulations containing the new nucleic acid as active ingredients and methods of treatment are also provided. The invention also provides a description of the physicochemical properties of this novel form of Rugged dsRNA and a method for its preparation in substantially pure form. DsRNAs acting thru TLR3 receptor activation are potent antiviral compounds as well as anticancer agents; also through secondary immunomodulation they can enhance the bioactivity of vaccines and also treat autoimmune disorders.