Amniotic fluid cells (AFC) from 10 women undergoing amniocentesis were investigated. The 5-bromo-2'-deoxyuridine-induced sister chromatid exchange (SCE) frequency of AFC remained stable after the addition of a therapeutical concentration of Viscum album L. preparation Iscador P but decreased significantly after administration of high drug doses. As the proliferation index remained stable, even at extremely high drug concentrations, this effect could not be ascribed to a reduction of proliferation. No indications of cytogenetic damage or effects of mutagenicity were seen after the addition of Viscum album L. preparation P.