Purified histone H1 exerts extracellular functions suggesting novel histone functions. The cytotoxic effects of histone H1 have lead to its choice as a pharmacological tool in breast cancer. Hence the present study was aimed at investigating the effect of exogenous histone H1 on the proliferation of estrogen receptor positive (MCF 7) and estrogen receptor negative (MDA MB 231) human breast cancer cells. Cells were incubated with various concentrations of histone H1 and antiproliferative activity was assessed by MTT assay. Proliferation of breast cancer cells was assessed from the activity of ornithine decarboxylase (ODC) using [14C] labeled ornithine. Histone H1-mediated cellular effects, such as anchorage dependent growth and apoptosis, were assessed by colony formation assay, fluorescence microscopy after acridine orange/propidium iodide staining and DNA fragmentation analysis. Histone H1 was significantly cytotoxic as it inhibited colony formation, ODC activity and induced apoptosis in both estrogen receptor positive and estrogen receptor negative cells. These results suggest that histone H1-induced antiproliferative effects on human breast cancer cells could possibly involve inhibition of ODC.