To maximise sensitivity, protocols for testing chemi-assumptions to be tested. To date, the kinetics of cals in chromosomal aberration assays in vitro are induction and dose -response relationships of recipdesigned so that cells are sampled when the peak rocal translocations induced by chemicals have frequency of aberrations might be expected to oc-been little investigated. We compared the frecur. They are not designed to measure the fre-quency of chromosome-type aberrations in human quency of aberrations in cells which survive. Only lymphocytes following treatment with two powerful chromosomal aberrations which are heritable, how-clastogens, streptonigrin and Trenimon, using conever, can have any relevance to human health, but ventional staining techniques and chromosome the detection of those aberrations most likely to be painting. The results show that although reciprocal tolerated (inversions, reciprocal translocations) is translocations can be shown to arise and persist notoriously difficult with conventional light micros-in treated populations of human lymphocytes for copy. Current protocol design is justified by arguing several days following treatment, their frequency is that the presence of structural aberrations of any very low, even at concentrations where large type at early times after treatment indicates a risk amounts of chromosomal damage are induced, inthat a proportion of aberrations will persist and be dicating that, at present, the value of using chromomaintained in the population. Chromosome paint-some painting as an adjunct to traditional clastoing allows reciprocal exchanges to be relatively genicity testing is limited. Environ. Mol. Mutagen. easily measured and permits the validity of these 32: 212 -222, 1998