The composition of nuclear proteins from human thymocytes and T lymphocytes from peripheral blood was analyzed. Total thymocytes and total peripheral blood T lymphocytes differed markedly in non-histone chromatin proteins (both phosphorylated and non-phosphorylated), but did not differ in histones. When the cells were separated according to density, T-lymphocyte fractions with a close specific gravity showed restricted differences in non-histone chromatin patterns.