In previous papers (Del Giudice et al. Curr Genet 8:493-497, 1984; Massardo et al. Curr Genet 17:455-457, 1985) we have shown that strains of Saccharomyces cerevisiae that are devoid of mitochondrial DNA (rho(o)) are resistant to the alkaloid lycorine isolated from Amaryllis plants, whereas strains containing mitochondrial DNA (rho(-), mif(-), or rho(+)) are sensitive to this drug. In addition, we were able to show that the so-called hypersuppressive petites, whose mitochondrial genomes consist of short regions of DNA containing an ori sequence, show intermediate resistance. In this paper, we demonstrate that the degree of suppressiveness of a rho- mutant correlates with the degree of resistance to lycorine.