Missense errors in the translation of mRNAs in Saccharomyces cerevisiae were screened by looking for charge heterogeneity of proteins on two-dimensional gels resulting from the substitution of charged and neutral amino acids. No such mistranslation was detected in wild-type yeast strains grown in the presence of the translational error-inducing antibiotic paromomycin. However, paromomycin-induced mistranslation of a heterologous mRNA, encoding human phosphoglycerate kinase expressed in yeast, was seen. We suggest that the combination of error-prone translation of a heterologous mRNA, and growth in the presence of paromomycin, leads to an accumulation of mistranslated proteins that can be detected by two-dimensional gel electrophoresis.