Chromosome replication at the beginning and termination of the S period in a primary culture of embryonic human fibroblasts has been studied through the use of 3H-thymidine labeling and autoradiography. In the analysis the observed numbers of silver grains above chromosomes were compared with those expected on the assumption of uniform labeling throughout the whole chromosome complement. The S period began almost simultaneously in all chromosomes but No. 1 (in which synthesis began very early) and the Y (in which it began late). The chromosomes terminate their replication asynchronously; Nos. 2, 3, 4–5 and 19–20 terminate DNA synthesis early, while Nos. 1, 16, 21–22 and the Y replicate late. The results obtained have been compared with published data on DNA replication in cultured peripheral blood leucocytes. The differences found in chromosome replication (especially in that of Nos. 1 and 2) may be due to differences in the function of these chromosomes in the two cell types.