We have constructed ideograms of human prometaphase chromosomes from synchronized and from standard 72-h lymphocyte cultures. G banding was achieved by a trypsin-Giemsa (or Wright's stain) method. In addition to light (white) and dark (black) bands, we have distinguished three different shades of grey. This distinction is essential for proper identification of the increasing number of bands displayed by high-resolution chromosomes. The relative amount of chromatin in each category of staining intensity has been calculated and expressed as 'light value.' The ideograms represent the maximal number of bands discernible with some consistency on prometaphase chromosomes, i.e., 721 euchromatic and 62 'variable' heterochromatic or heteromorphic bands. The ideograms are based on measurements. On selected printed copies of each chromosome derived from different cells and different individuals, the relative width of each band was measured in relation to the length of the respective chromosome arm. The measurements per chromosome were averaged and used for construction of the ideograms. The distance of each border between bands or sub-bands from the centromere has been calculated on a relative scale, with positions 0 at the centromere and 1.0 at the p terminus of q terminus. The numbering system for bands and sub-bands follows the Paris Conference (1971) recommendations.