Autoradiographic studies of human chromosomes labeled with tritiated thymidine have been carried out on cells from 15 individuals with normal and abnormal karyotypes. The labeling method that was applied is based on the observation that, in the late part of the DNA synthetic period (S-period), specific, large chromosome segments complete DNA replication at different time intervals. The DNA replication patterns are reported as they were observed, for individual chromosome pairs, during the last two hours of the S-period. It is demonstrated that the method has become a useful tool for chromosome identification and characterization, and a detailed description of the techniques used, is given. Secondary constrictions in certain chromosome pairs which have been demonstrated by means of special techniques (Sasaki and Makino, 1963; Saksela and Moorhead, 1962) are shown to coincide with specific, late DNA replicating chromosome regions.