Effects of Hoechst 33258 on human leukocytes in vitro

The benzimidazole derivative Hoechst 33258 was added at various concentrations to human leukocyte cultures. After 16 or 24 h of treatment, with concentrations equal to or greater than 100 mug/ml of Hoechst 33258, a number of chromosomes showed regions in which the chromatin was undercontracted. The centromeric regions of chromosome 1 and, more rarely, of chromosomes 3 and 9 appeared to be decondensed. Short decondensed regions were also present on the long arms of chromosomes 1 and 2. The possible nature of these regions is discussed.