Populations from seven pairs of neoplastic and nonneoplastic cell lines of common origin were compared for evidence of a change in morphotogical parameters accompanying neoplastic transformation. Projected cytoplasmic area, projected nuclear area, cell dry mass, nuclear dry mass, whole-cell absorption at 265 nm, and nuclear absorption at 265 nm were determined. Neoplastic transformation was consistently accompanied by three morphological features: (a) decrease in projected area of the lamellar cytoplasm; (b) decrease in the projected area of the nucleus; and (c) decrease in dry mass of the lamellar cytoplasm. Ultraviolet absorption was generally less in the neoplastic cells than in the nonneoplastic cells, while nuclear mass remained approximately the same. These results are interpreted as quantitative evidence that neoplastic transformation is accompanied by morphological change. The morphological events may be characterized as a loss in cell surface and nuclear membrane components.