[Late replicating human X-chromosome in the early S-period]

The early S-period in cultured human lymphocytes was studied using the premature chromosome condensation technique. In the lymphocytes, isolated from females but not from males, one uncondensed G1-chromosome was detected among "pulverized" chromosomes which remained unlabeled with H3-thymidine. This chromosome is regarded as an inactivated X-chromosome. The data obtained are in agreement with the phenomenon of the delayed onset of DNA replication in the inactivated X-chromosomes.