From observations on the partial disintegration of mitotic chromosomes isolated from human fibroblasts we propose that human mitotic chromatids are characterized by a rather simple organization based on the folding and coiling of a long, regular, hollow cylindrical structure the unit fibre , with a diameter of about 400 nm (Bak, Zeuthen & Crick 1977). This structure is postulated to consist of a super-solenoid formed by the coiling of the 30 nm solenoid (itself formed by coiling of the string of nucleosomes).