Cells containing the mouse T(X;7)1Ct translocation were used to separate active and inactive X chromosomes. Cytophotometric data showed that the X(7) chromosome was at least 15% larger than any normal mouse chromosome. Cloned cell lines were established from male and female embryos with a normal karyotype and carrying the X(7) chromosome in an active or inactive state as shown by replication kinetics. As expected from the cytophotometric data, flow histograms of normal mouse chromosomes stained with 33258 Hoechst had five peaks. A sixth peak was present when the chromosomes came from mice carrying T1Ct. In this peak 6, 30 to 50% of the chomosomes were X(7) and there was almost no contamination by the normal X chromosome.