Nuclei of the human breast epithelial cells, MCF-10A, transfected with the c-Ha-ras oncogene, the ras proto-oncogene and the plasmid Homer 6 only, were studied by image analysis after Feulgen staining. This material had been previously used to demonstrate that the experimental insertion of the activated c-Ha-ras oncogene into the DNA of the MCF-10A cells induces their tumoural properties. The ras-transformed nuclei of the MCF-10A cells exhibited differences in chromatin supraorganization in comparison with the nuclei of human breast carcinoma MCF-7 cells, or c-Ha-ras-transformed NIH/3T3 cells and, to a much lesser extent, with those of other MCF-10A transfectants and the non-transfected MCF-10A cells. All MCF-10A transfectants exhibited unravelling of both condensed and non-condensed chromatin, which, however, was less drastic in the ras-transformed MCF-10A cells. It is hypothesized that simultaneous to a general chromatin loosening as a response to foreign transfected DNA, a reverse mechanism may be elicited by ras transformation in the chromatin of the MCF-10A cells. The result in terms of elicited chromatin condensation was not as strong as that promoted in ras-transformed NIH/3T3 cells. Considering that early steps of tumour progression in vitro have previously been assumed to be involved in the ras-transformed MCF-10A cells, the differences in chromatin supraorganization of the ras-transformed MCF-10A cells as compared with MCF-7 cells are probably due to their different tumoural stages plus the putative effect of the transfected DNA vector on the transfected MCF-10A cells.