Summary In vitro beryllium (Be) binding to rat liver nuclei has been reassessed ( K Ass = 2.0 × 10 6M: n = 17 nmol Be/mg protein). Be also binds to rat liver nucleoli ( K Ass approx. 4 × 10 6 M: n = 10 nmol Be/mg protein). Examination of rat liver chromatin fractionated on a hydroxyapatite column shows that Be does not bind to histone or to the non-histone protein eluted by 0.05 M sodium phosphate. Be is strongly bound to the non-histone proteins eluted by 0.2 M sodium phosphate ( K Ass = 1.1 × 10 6M: n = 55 nmol Be/mg protein) and also to the same extent to the fraction containing DNA which is subsequently eluted from the column. Evidence is provided that the latter binding is not due to DNA. The fractions containing the Be-binding proteins also contain the proteins which are phosphorylated to the greater extent.