The histone, neutral and acidic nuclear-protein content of BR6 mouse mammary tumours have been measured and related to growth rate and hormone responsiveness. In unresponsive tumours, histone and neutral nuclear-protein content was inversely correlated with growth rate while acidic nuclear-proteins were directly correlated with growth rate. The histone content of these tumours was low when compared to pregnancy dependent tumours growing at the same rate under the same physiological conditions.In vitro incorporation of labelled amino acids into total protein and the nuclear proteins was also measured. Pregnancy independent tumours had higher rates of total protein synthesis than pregnancy dependent tumours The rates of incorporation of amino acids into the nuclear proteins also tended to be higher in independent tumours.