Some of the properties of three continuous cell lines derived from BK virus-induced hamster tumors were examined. The cell lines had in vitro growth characteristics of transformed cells. Morphologically most of the cells were fibroblastic, but multinucleated giant cells were also common. Ultrastructurally all three cell lines displayed the usual features of cells grown in vitro. Marked variation in the nuclear size and shape as well as prominent nucleoli were characteristic to these cells. No viruses or virus-like particles were found. Virus isolation attempts by fusing the cells with Vero cells were negative, and no virion antigen was detected in these cells by immunofluorescence. T antigen similar to that of other papovaviruses was found in the cells. This antigen stained with sera from a number of hamsters carrying transplanted BK virus-induced tumors, and also with SV 40 T antisera. The antigen disappeared after 30 minutes at 56 degrees C. Cytogenetic analyses showed that the three cell lines were heteroploid with subtetraploid numbers of chromosomes. Chromosome abnormalities were also seen. All three cell lines induced sarcomatous tumors in adult hamsters after subcutaneous inoculation.