Among the mitochondrial conditional mutations localized in the gene coding for the 21S ribosomal RNA, one--ts 902--produces severely reduced amounts of 21S RNA and 50S subunit. We investigated its physiological properties and found that this thermosensitive mutation was associated with highly pleiotropic effects. The mutant phenotype is associated with cell death in certain conditions, and with a massive accumulation of rho- mutants at non-permissive temperature. Furthermore, interactions with the sites of action of erythromycin and chloramphenicol, both localized within the 21S rRNA, were detected. The mutant is hypersensitive to erythromycin and has a cis-incompatibility with the chloramphenicol-resistant mutation CR321. Ts 902 thus appears to have a dual effect, not only at the ribosomal level but also at a cellular level.