The results of the present study demonstrate once again that, on the average, the addition of LSD in vitro leads to chromosome damage in excess of that observed in cultures without such added LSD even though nearly all of the in vitro experiments were carried out on blood cultures derived from patients who had already been started on the drug regime in vivo. By contrast, the present data provide no evidence for a measurable detrimental effect of LSD and DA when administered to patients under medical supervision. It is conceivable that an effect might have emerged had we been able to monitor chromosomes separately before and after administration of LSD and before and after administation of DA, but we consider it unlikely that either drug would exert protective action against potentially damaging consequences of the other. It is conceivable also that damaged cells are sequestered to give rise eventually to neoplasia-prone clones. The likelihood of that possibility can be determined only by long-term follow-up studies.