A mutant of Saccharomyces cerevisiae deficient in the lactate-proton symport was isolated. Transformation of the mutant with a yeast genomic library allowed the isolation of the gene JEN1 that restored lactate transport. Disruption of JEN1 abolished uptake of lactate. The results indicate that, under the experimental conditions tested, no other monocarboxylate permease is able to efficiently transport lactate in S. cerevisiae.