Histone H1(0) is a linker histone subvariant present in tissues of low proliferation rate. It is supposed to participate in the expression and maintenance of the terminal differentiation phenotype. The aim of this work was to study histone H1(0) distribution in human breast carcinoma and its relationship with the processes of proliferation and differentiation. Most of the cells in carcinomas of moderate and high level of differentiation expressed histone H1(0) including cells invading connective and adipose tissues. In low differentiated tumours, the number of H1(0) expressing cells was considerably lower. Staining of myoepithelial cells, when seen, and of stromal fibroblasts was variable. The metastatic malignant cells in the lymph nodes also accumulated H1(0) but lymphocytes were always negative. All immunopositive malignant cells exhibited signs of polymorphism. Double H1(0)/Ki-67 staining showed that the growth fraction in more differentiated tumours belonged to the H1(0)-positive cells, while in poorly differentiated carcinomas it also included a cell subpopulation not expressing H1(0). If expressed, p27Kip1 was always found in H1(0)-positive cells. These findings are inconsistent with the widespread view that histone H1(0) is expressed only in terminally differentiated cells. Rather, they suggest that the protein is expressed in cells in a prolonged intermitotic period irrespective of their level of differentiation. Double H1(0)/Ki-67 immunostaining could be a useful tool in studying the growth fraction in tumours.