The present invention provides for the intratumoral delivery of at least one immunostimulatory cytokine in combination with at least one checkpoint inhibitor. In particular, it provides delivery of a plasmid encoding the immunostimulatory cytokine using intratumoral electroporation. The checkpoint inhibitor may be administered systemically or encoded on a plasmid and delivered using intratumoral electroporation. The checkpoint inhibitor may be delivered contemporaneously with or after treatment with the immunomodulatory cytokine.