The interactions of DNA with the five histone components (H1, H2B, H2A, H3 and H4) and with a number of histone fragments (N-H1 (1-72), C-H1 (73-216), N-H2B (1-59), C-H2B (63-125), N-H2A (1-39), C-H2A (58-129), N-H4 (1-84) and C-H4 (85-102) have been studied by using the techniques of thermal denaturation and solubility behaviour. Complexes in 10-3 M phosphate buffer, 2 · 10-5 M Na2-EDTA, pH 7.0 were prepared by the direct mixing method. For lysine-rich histones (H1 and H2B) it has been found that the main characteristics which governs the interaction with DNA are located in the very lysine-rich part of the molecules, i.e. in the C-H1 and N-H2B segments. These regions are also responsible for a cooperative distribution of the histone along the DNA molecules in the artificial complexes. It appears from our studies that the tertiary structure of the moderately, arginine-rich histone (H2A) is an essential feature for its interaction with DNA. The two arginine-rich histones (H3 and H4) complexed with DNA behave in a similar way, both in thermal denaturation and in DNA precipitation. In the case of C-H4, a marked shift of the melting profile has been observed which is correlated with the presence in the peptide of the hydrophilic cluster Lys-Arg-Gln-Gly-Arg-Thr. Out results suggest that large segments rich in lysine and basic clustering within histones give rise to different modes of electrostatic interaction with DNA. © 1977.