X inactivation studies have been carried out on lymphocytes from eight unrelated females heterozygous for the Martin-Bell syndrome. Four of these carriers were of normal IQ and four were mentally handicapped. When BrdU was used to differentiate between the active and inactive X chromosome an average of 55% of fra(X) were active in the retarded subjects, but only 27% were active in those of normal IQ. When 3H thymidine was used to differentiate between the active and inactive X chromosome, an average of 58% of mitoses from handicapped subjects and 33% of mitoses from normal subjects showed an active fra(X) in informative cells. These results are compared with previously published studies and it is concluded that the number of inactive fra(X) chromosomes calculated as a proportion of all cells scored is the same in mentally normal and mentally retarded subjects. However, the number of active fra(X) chromosomes is consistently higher in the retarded than in the normal females.