The yeast non-Mendelian factor [psi(+)] has been suggested to be a self-modified protein analogous to mammalian prions. Here it is reported that an intermediate amount of the chaperone protein Hsp104 was required for the propagation of the [psi(+)] factor. Overproduction or inactivation of Hsp104 caused the loss of [psi(+)]. These results suggest that chaperone proteins play a role in prion-like phenomena, and that a certain level of chaperone expression can cure cells of prions without affecting viability. This may lead to antiprion treatments that involve the alteration of chaperone amounts or activity.