The present invention discloses lentogenic viral strains useful in the treatment of cancer. A preferred viral strain of Newcastle Disease Virus (NDV) is specifically characterized in terms of biological activities. The present invention further discloses treatment of cancer by application of a clonal NDV strain to tumors. Another feature of the invention is the use of at least one isolated viral glycoprotein or a subunit or analog thereof or of an isolated polynucleotide encoding the same for the treatment of cancer.