Oncolytic virotherapy is a promising approach for the treatment of cancer patients. Non-pathogenic strains of single-strand RNA containing paramyxoviruses, such as avian Newcastle Disease Virus and Measles virus, are being widely tested as candidates for oncolytic therapeutics. Non-pathogenic for humans mouse Sendai virus that belongs to the Paramyxoviridae family. The virus had being testing in pilot trials in the 1980-s for cancer treatment in Russia with promising results. Sendai virus requires exogenous host proteases for the maturation of its virions, which occurs by a proteolytic processing of the viral envelope fusion (F) protein. Therefore, multiple rounds of virus replication in cell culture require the addition of exogenous trypsin to culture medium. We tested the ability of several different human cancer cell lines to support multiple rounds of replication of the Sendai vires and to process the F protein without the addition of exogenous trypsin. We found that unlike the control MDCK cells that do not secrete mature infectious virions, most of the human cancer cell lines tested were capable of supporting the replication of mature virus. The mechanism for the acquired permissiveness of cells to Sendai virus infection is explained by activation of expression of host-cell transmembrane proteases that apparently process the Sendai virus F protein during the budding of newly replicated virions. We suggest that frequent activation of host cell proteases may substantially contribute the oncolytic activity of Sendai virus.