The hypothesis of tumor progression proposed by Nowell [P. C. Nowell, Science (Wash. DC), 194: 23-28, 1976] states that one mechanism for the development of the metastatic phenotype could be the induction of chromosomal instability. We have developed a new experimental system for studying the induction of the metastatic phenotype using early passage fibroblasts which become metastatic in nude mice after transformation with the Harvey ras oncogene [R. J. Muschel et al., Am. J. Pathol., 121: 1-8, 1985; R. Pozzatti et al., Science (Wash. DC), 232: 223-227, 1986]. Since the early passage fibroblasts themselves are diploid, we reasoned that this might be a system in which karyotypic change after tumor formation or metastasis might easily be evaluated. Thus, we performed cytogenetic analysis on multiple metastases and tumors which had been derived from cells transformed with the cellular Harvey ras1 oncogene and compared their karyotypes. The karyotypes of the cells isolated from 5 tumors and 14 metastases were, as far as we could determine, identical to those of the injected cells. This could easily be evaluated because of the two clones studied one was diploid; the other has a trisomy of chromosome 4 without any other detectable abnormality. These results suggest that in this system using nude mice, selection for a necessary or even advantageous chromosomal aberration does not occur during tumor formation or metastasis. Furthermore, they indicate that the presence of the ras gene itself does not induce chromosomal rearrangements or aneuploidy and that a cell can be both tumorigenic and metastatic yet remain diploid.