Many cell lines used for vaccine production have a potentially strong tumorigenic character. Some of those routinely used need to be checked at different passage numbers for this characteristic. Using HeLa cell cultures as positive controls, and primary canine kidney cell (CKC) or feline kidney cell (FKC) cultures purified in vitro on passage three as negative controls, the tumorigenicity of VERO cell sublines was tested in 219 nude mice. The master cell stocks (MCS) and working cell banks (WCB) of eight strains of VERO African green monkey kidney cell (AGMKC) line used for canine, feline and mink vaccine preparation were established in China. The hypo-tetra-ploid JA or hyper-diploid KA strain of VERO line was highly tumorigenic. These data showed a variable chromosome karyotype of VERO line, and contraindicated the use of JA or KA strain of VERO line for the preparation of attenuated viral vaccines. JA or KA strain of VERO line could be a substitute for HeLa line as a positive-control malignant tumor (MT) cell model. The non-carcinogenic YB, JC, M and JB strains of VERO line were therefore selected for the preparation of modified live rabies viral vaccine in place of BHK-21. The cell sub-lines are comparatively stable in terms of their heritable characters, and show little significant changes between passages. In summary, we have found that: 1) the tumorigenicity of cell line is different among different-karyotypic cells; 2) it is the genetic characteristics of chromosomes of cell lines that determines their tumorigenicity, but with species-specific carcinogenicity; 3) the chromosome number variation of cell lines has positive relationship with their carcinogenesis; 4) highly variable strains of tumor cell line can be selected quickly and successfully in nude mice by alternate cultivation in vitro and in vivo. Malignant rhabdoid tumor (MRT) was evolved in nude mice inoculated with violently variable HeLa or VERO cells. The importance of assessing the tumorigenicity in cell sublines used for vaccine production is emphasised.