Three X/autosomal translocations, two familial and one de novo, were analyzed. Late-replicating chromosomes and chromosome regions were studied with R-banding techniques after BrdU incorporation. The first translocation, t(X;4)(q21;ql3), was a de novo translocation, found in a woman with amenorrhea. The structurally normal X was late replicating in all cells. The second translocation, t(X;6)(p21;q26), was found in an unbalanced form, 46, XX, der(6), in a phenotypically abnormal girl; her mother carried the balanced translocation. In the mother’s blood culture inactivation of the X’s followed two modes: In 85 % of the cells the normal X was late replicating, and in the remaining 15 % the der(X) was inactivated, including the attached fragment of chromosome 6. The third translocation, t(X;17)(pll;q24), was found in three generations. In the phenotypically normal mother, who carried the balanced translocation, the late-replicating X was always the normal X. In her daughters, who had an unbalanced karyotype, 46, X, der(X), and multiple congenital abnormalities, the X part of the translocation chromosome was always late replicating. No spreading of inactivation over the attached autosomal region was observed, resulting for these patients in a partial trisomy of 17q. Their peculiar phenotype is descri