The morphonuclear characteristics (nuclear size and chromatin pattern), the proliferation index and the ploidy level were characterized in a series of 46 breast tumors including medullary (5 cases), papillary (6 cases), lobular (27 cases), colloid (4 cases) and comedo- (4 cases) carcinomas. The quantitative assessments were carried out by means of digital cell image analyses of Feulgen-stained nuclei from imprint smears. The results show that monovariate analyses (one-way variance analyses) were much less potent than multivariate analyses (principal components analyses followed by the canonical transformation of the data and discriminant analyses) in assessing the morphonuclear characteristics of these breast tumors. The multivariate analyses indicated that there might be a level of malignancy which increases according to the sequence papillary and medullary and colloid carcinomas-->comedocarcinomas-->lobular carcinomas. This assertion is corroborated by the ploidy-level-related results which revealed a higher proportion of highly aneuploid cases in the group of lobular carcinomas than in the group which included medullary, papillary and colloid carcinomas. However, since highly aneuploid cases were also encountered in this latter low malignancy level group, we expressed the hypothesis firstly that aneuploidy reflects two distinct biological properties, i.e. the aggressiveness of a tumor and its age, and secondly that a highly aneuploid but low malignancy tumor should correspond to old degenerating tumors.