Alterations in nucleolar morphology and number have been routinely noted by pathologists and tumor biologists. More recently, cancer patient prognosis has been correlated to increases in nucleolar size, number and morphology in tumor sections. This study investigated the nucleolar changes in the well-characterized SV40 T antigen transformation model by assessing B23-positive nucleoli. B23, or nucleophosmin, is essential in rRNA processing and p53 regulation via the sequestration of p19ARF. By using B23 immunofluorescence, we found that cells expressing T antigen manifest significant increases in the number, size and the number of large nucleoli present. The total number of nucleoli is increased significantly in both spontaneously immortalized C57Bl/6 mouse embryo fibroblasts and T antigen transformed cell lines compared to the primary cells; however, the properties of growth rate, saturation density and tumorigenesis correlated to significant increases in nucleolar size, not the total number of nucleoli.