Chloramphenicol resistance in mammalian cells is cytoplasmically inherited. In yeast, a similar phenotype is caused by mutations in the mitochondrial DNA (mtDNA), and sequencing of carefully constructed strains has identified nucleotide monosubstitutions in the 3' region of the large (21S) rRNA gene which correlate with the antibiotic resistance. We have sequenced the corresponding section of mammalian mtDNA from chloramphenicol-resistant cell lines for comparison with the wild-type sequence. Differences between the sequences occur at positions similar to those altered in the yeast mutants, in a highly conserved region of the large (16S) rRNA gene.