Five beta-blockers (propranolol, metoprolol, oxprenolol, pindolol, and sotalol) were tested for their in vivo DNA-damaging activity by two different techniques: alkaline denaturation of DNA followed by hydroxylapatite chromatography, and a new viscometric method markedly more sensitive in detecting DNA fragmentation. DNA damage, as checked by the first technique, was absent either in liver and kidney of mice or in liver and gastric mucosa of rats given a single p.o. administration of 1/2 LD50 of the drugs. The subsequent viscometric analysis of liver DNA from rats treated with the same doses confirmed the above negative results.