Observations have been performed on peripheral blood lymphocytes from a patient having ingested 200 mg thallium sulfate, in order to evaluate the ability of the compound to produce cytogenetic damage in vivo in humans. Our results demonstrate that neither the yield of structural chromosome aberrations nor sister chromatid exchanges were significantly modified. The drastic increase of binucleated cells with micronuclei indicates that thallium sulfate has in common with many metallic compounds the ability to interfere with chromosome distribution.