For Tetrahymena thermophila cells to survive extended periods of time at 43 degrees C, they must continuously synthesize heat shock proteins. For its translational machinery to function at 43 degrees C, T. thermophila requires either prior nonlethal heat shock treatment or brief treatment with partially inhibiting doses of cycloheximide or emetine. We have identified and characterized a mutant strain of T. thermophila (MC-3) in which prior nonlethal heat shock does not prevent protein synthesis inactivation at 43 degrees C. In addition, treatment of MC-3 cells with either of the antibiotics that normally confer 43 degrees C thermoprotection on wild-type cells elicited no similar thermoprotective response in these cells. Despite these phenotypic characteristics, by other criteria MC-3 synthesized a normal, functional array of heat shock proteins at 40 degrees C, a nonlethal heat shock protein-inducing temperature. The mutation in MC-3 which prevents the thermostabilization of protein synthesis by nonlethal heat shock is, by genetic criteria, most likely the same one which prevents the induction of thermotolerance by drug treatments. We present evidence that this mutation may affect some ribosome-associated functions.