The inheritance and phenotype of multiple drug resistance in independent multiple drug resistant mutants, two isolated in this laboratory (GR359 and 2-20), and two (DRI 9/T7 and DRI 9/T8) reported by Guerineau et al. (Biochem. Biophys. Res. Commun. 61,462), was investigated. Comparison of resistance to 12 selected drugs showed that the resistance phenotypes of all mutants were similar, although some differences in levels of resistance of each mutant was observed with certain drugs. Mapping of the resistance loci in GR359 and 2-20 revealed tight linkage of both resistance genes to the centromere linked gene leul. 2 micron DNA was analysed by hybridization of 2 micron RNA to EcoRI fragments of a total DNA extract. Eight partial revertants of 2-20, which had been chosen as having a phenotype similar to the 2 micron DNA deficient [cir degrees] isolate DRI 9/T7, revealed the presence of 2 micron DNA. The lack of detectable 2 micron DNA in DRI 9/T7 was confirmed.