ABSTRACT X-chromosome inactivation was investigated cytologically using the modified Katida method which differentially stains inactive X-chromosome material at metaphase in balanced-day female embryos heterozygous for four X-autosome rearrangements, reciprocal translocations T (X; 4) 37H, T (X; 11) 38H and T (X; 16) 16H (Searle’s translocation) and the insertion translocation Is (7; X) 1Ct (Cattanach’s translocation). In all cases non-random inactivation was found. In the reciprocal translocation heterozygotes only one translocation product ever showed Kanda staining. In addition in a proportion of cells from T (X; 4) 37H, T (X; 11) 38H and Is (7; X) 1Ct the Kanda staining revealed differential staining of X-chromosome material and attached autosomal material within the translocation product. In a study of-day female embryos doubly heterozygous for Searle’s translocation and Cattanach’s translocation two unbalanced types of embryo were found. In one type of unbalanced female embryo of the karyotype 40 (X (7)/X16; 16/16) no inactivated X-chromosomal material is found. A second unbalanced type of female embryo, of the presumptive karyotype 40 (X (7)/XN; 16x/16) was found in which two inactivated chromosomes were present in the majority of metaphase spreads. A simple model for the initiation of X-chromosome inactivation based on the presence of a single inactivation centre distal to the breakpoint in Searle’s translocation explains these findings.