Early-, mid- and late-passage cultures (population doubling levels 12, 35, and 51, respectively) of IMR-90 fibroblasts were exposed to 3H-thymidine for 48 h prior to fixation in situ for morphometric analysis in order to determine quantitatively what ultrastructural changes accompany the loss of proliferative capacity during aging in vitro. Analysis of autoradiographs, both at the light and electron microscopic levels, with an image analyzer followed by ANOVA statistical scrutiny demonstrated that a significant increase in relative cell area, an indicator of cell size, was characteristic of cells unable to incorporate 3H-TdR at both mid- and late-passage, but not at early-passage levels. Nuclear size also increased significantly with progressive passage level but was not related to proliferative capacity. No significant difference in the area fraction of nucleoli per unit area of nucleus or of mitochondria, Golgi, or lysosomes was seen in either subpopulation at any passage level. Dilated cisternae of rough endoplasmic reticulum in early-passage cells were seen if cells were harvested with trypsin and fixed either before or after centrifugation, but were not seen in labeled or unlabeled cells from any passage level when cultures were fixed in situ. We conclude that a significant increase in cell size is the only significant morphological change associated with the loss of proliferative capacity of IRM-90 fibroblasts. Furthermore, our data indicate that there is no accumulation of secondary lysosomes in human diploid fibroblasts during aging in vitro; we therefore cannot support any hypothesis of aging or proliferative decline that is based mechanistically upon this phenomenon.