Clonal strains of vaccinia viruses are provided. Also provided are methods of identifying and isolating attenuated and oncolytic clonal strains from virus preparations, Modified recombinant forms of the clonal strains also are provided. The clonal strains and virus preparations can be used for diagnostic and therapeutic methods, in particular for therapy and diagnosis or monitoring treatment of proliferative disorders, including neoplastic diseases, such as, but are not limited to, solid tumors and blood cancers.