It is unknown how and why the genetically inactivated mammalian X chromosome replicates late in S phase. There are also occasional inactive X chromosomes characterized by an opposite behavior replicating early in S phase. Two clonal cell lines, MTLB3 and MTLH8, isolated from a cultured murine T-cell lymphoma have an allocyclic X chromosome of the latter type. This precociously replicating X chromosome was judged to be genetically inactive as the late replicating one. Immediately after fusion with another cell line, the precociously replicating X chromosome from these cells starts to replicate late in S phase. This finding seems to suggest that late replication characterizing the inactive X chromosome is actively maintained by a trans-acting factor in female somatic cells, and that its lack entails a switch from late replication to precocious replication. It remains unknown whether this presumptive factor also modifies the autosomal replication pattern.