A Neurospora spheroplast assay was used to study the cytotoxicity and mutagenicity of cis platin (cis dichlorodiammine platinum II) in a eukaryote. Mutagenicity was measured by using reversion of several alleles of the am (NH3 assimilatory glutamate dehydrogenase) gene. A mutation (uvs-2) that disables a DNA repair pathway in Neurospora resulted in reduced sensitivity to and elimination of mutability by cis platin. The implications for other eukaryotic systems (including human cells) are discussed.