With the aim of evaluating the effect of oxamniquine, a drug used in the clinical trials of schistosomiasis mansoni, on chromatin of animal cell nuclei, the patterns of nuclear basophilia and anisotropy were determined for hepatocytes, cardiac miocytes, and lymphocytes of young adult mice. The animals received the oxamniquine orally in a single dose of 436 mg/kg. Cell preparations were obtained at various times after the administration of the drug. The nuclei were subjected to RNAse digestion and stained with a toluidine blue solution at pH 4.0. No difference was found when comparing the patterns of nuclear basophilia and anisotropy of treated animals with those of untreated controls. It was demonstrated that the oxamniquine molecules do not penetrate the interior of the DNA double helix, neither do they alter the helical conformation of this nucleic acid. Furthermore, the oxamniquine molecules do not attach themselves to the free phosphate groups of the DNA. Therefore, this drug does not affect the DNA of the animal chromatin, differing from another schistosomicide, hycanthone.