The DNA values, in arbitrary units, have been measured in human fibroblast-like cells grown in tissue culture. Three cultures were available. Each cell nucleus was further classified according to the presence or absence of Barr bodies, or the presence of multiple chromocentres. Nuclear areas were also measured. The DNA values of all cultures fell into two main classes, which were interpreted to represent nuclei before and after DNA synthesis respectively. A smaller number of cells with intermediate DNA values were presumed to be in the process of DNA synthesis. It was found that the incidence of Barr bodies was higher in cells which had completed DNA synthesis than in those with the basic DNA value. In cells with intermediate DNA values, the incidence of Barr bodies was higher than in cells with the basic DNA values but lower than in cells which had completed DNA synthesis. These results are in agreement with those of other workers who found that the Barr body is not absent throughout DNA synthesis. In all DNA classes, nuclei with multiple chromocentres were the smallest, and cells with a single Barr body were smaller than those lacking a Barr body. The mean nuclear areas rose stepwise with each DNA class, although there was considerable intra-class variation.