We have investigated differences between the actions of encephalomyocarditis virus (EMCV) on cytometric indices in cultured NIH 3T3 and HEp-2 cells, which are characterized by different levels of transformation. HEp-2 cells surviving 48 h after EMCV infection showed lower nuclear ploidy, reduced nuclear area, fewer nucleoli and a higher percentage of euploid cells. There was a significant increase of nucleolar/nuclear DNA 6–24 h after EMCV infection. However, EMCV had markedly different effects on NIH 3T3 cells: there was a consistent increase in population ploidy, but the average number of nucleoli and the number of euploid cells in the population remained constant. The nucleolar/nuclear DNA ratio was almost unchanged. These different viral effects might be explained by the contrasting levels of differentiation of the cultured cell lines. The number of nucleoli does not depend on the amount of nuclear DNA in either viral-infected or intact cells but on the euploidy-to-aneuploidy ratio. The ratio of the sums of the nucleolar perimeters to the nuclear perimeter increases linearly with the number of nucleoli per nucleus in both intact and virus-infected cells. In both cell lines, the amount of DNA per nucleolus decreases as the number of nucleoli increases.