The highly reiterated alpha DNA tandem repeat was extracted from HeLa cells incubated with (cyanomorpholino)adriamycin (CMA) using mild techniques and subsequently probed for drug adducts by exonuclease III. The sequence specificity of the CMA-induced blockages was compared with that for blockages induced on the same DNA fragment when reacted in vitro. The sequence specificity of the drug-induced blockages was the same on both the isolated and the intact cell alpha DNA templates, with blockages predominantly associated with GpG sequences on either strand of the DNA.