Sup35 and Sup45 are essential protein components of the Saccharomyces cerevisiae translation termination factor. Yeast cells harbouring the [PSI+] prion form of Sup35 have impaired stop codon recognition (nonsense suppression). It has long been known that the [PSI+] prion is not stably transmitted to daughter cells when yeast are grown in the presence of mM concentrations of guanidine hydrochloride (GuHCl). In this paper, Mendelian suppressor mutations whose phenotypes are likewise hidden during growth in the presence of millimolar GuHCl are described. Such GuHCl-remedial Mendelian suppressors were selected under conditions where [PSI+] appearance was limiting, and were caused by missense mutations in SUP35 or SUP45. Clearly, antisuppression caused by growth in the presence of GuHCl is not sufficient to distinguish missense mutations in SUP35 or SUP45, from [PSI+]. However, the Mendelian and prion suppressors can be distinguished by subsequent growth in the absence of GuHCl, where only the nonsense suppression caused by the [PSI+] prion remains cured. Recent reports indicate that GuHCl blocks the inheritance of [PSI+] by directly inhibiting the activity of the protein remodelling factor Hsp104, which is required for the transmission of [PSI+] from mother to daughter cells. However, the nonsense suppressor activity caused by the GuHCl-remedial sup35 or sup45 suppressors does not require Hsp104. Thus, GuHCl must antisuppress the sup35 and sup45 mutations via an in vivo target distinct from Hsp104. Copyright © 2003 John Wiley & Sons, Ltd.