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A mentally retarded woman with multiple congenital anomalies is described. Her fibroblasts had a normal female chromosome complement, but in virtually all lymphocytes there were only 45 chromosomes, one no. 6 and one no. 19 having joined with their short arms. The two terminal deletions may have been submicroscopically small. Whether they had caused the clinical anomalies remains an open question. The translocation chromosome had only the primary constriction of no. 19, although no deletion in the centromere region of no. 6 could be detected even with centric staining. This appears to be the fourth well analyzed case of a large translocation chromosome that Seems to be dicentric with one centromere inactivated. The point of union of nos. 6 and 19 sometimes showed very unusual features: conspicuous bending of chromatids, a change in their thickness from the thinner no. 19 to the no. 6 component, and (to be described in detail elsewhere) chromatid breakage separating the two components (about once in 730 cells). Conceivably, the original breaks may both have occurred within telomeres.