Diploid cultures of human fetal fibroblasts were treated with various concentrations of chlordiazepoxide and of diazepam, with exposure times ranging from 8 h to 4 days. At termination of treatment, the chromosomes of the cells were prepared and analyzed with respect to chromosome damage. In all experiments, treated cultures showed an incidence of chromosomal aberrations and percentages of aberrant cells that did not differ from the values obtained in untreated control cultures. © 1969.