In a previous paper (Barboro et al., 1993, Biophys. J. 65, 1690-1699) we have shown that cancer development in the resistant hepatocyte model of Solt and Farber is characterized by the progressive unfolding of the higher-order structure of chromatin. A possible functional role of decondensation phenomena in cell transformation cannot be ruled out. Genetic activation involves the relaxation of the superstructure of chromatin, which may be, at least in part, modulated by its interaction with the nuclear matrix. Moreover, recent observations suggest that gene expression can be stimulated by alterations in the organization of the cytoskeleton. Therefore, we have characterized the changes in composition that the nuclear matrix-intermediate filament complex undergoes during the evolution of rat hepatocyte nodules. Dramatic changes in the expression of both the nuclear matrix and intermediate filament proteins occur during transformation; they are, however, related in a different way to the stages of carcinogenesis. Several new nuclear matrix proteins appear in early nodules, isolated 9 weeks after initiation. The subsequent evolution of persistent nodules is also characterized by discrete changes in the composition. Thus, the new synthesis of nuclear matrix proteins reflects the emergence of successive cellular populations, in line with the recent finding that a subset of components of the nuclear matrix is cell type-specific. In contrast, intermediate filament proteins undergo continuing changes. A new keratin with apparent molecular weight of 39 kDa, analogous to human keratin 19, appears in early nodules, and its expression steadily increases up to the 32nd week from initiation; at the same time, the amount of the proteolytic fragments of keratins A and D increases sharply. These findings suggest that the inappropriate expression of keratin 19 may be involved in the epigenetic activation of new cellular programs, through the rearrangement of the cytoskeleton which in turn may perturb nuclear matrix function.