The Gal4, Gal80, and Gal3 proteins of Saccharomyces cerevisiae constitute a galactose-responsive regulatory switch for GAL gene promoters. The low cellular levels of these proteins have hampered mechanistic studies and limit the utility of the GAL gene promoters for high-yield production of endogenous and exogenous proteins. We have constructed two new vectors, pMEGA2 and pMEGA2-Delta URA3, that increase the level of the Gal4p-Gal80p-Gal3p switch proteins under conditions that preserve the Gal3p-Gal80p-Gal4p stoichiometries required for normal switch function. Cells carrying pMEGA2 show 15- to 20-fold more Gal4p and 30-to 40-fold more Gal3p and Gal80p than cells lacking pMEGA2. These high levels of Gal4p, Gal80p, and Gal3p do not perturb the integrity of galactose-inducible regulation. Cells that carry pMEGA2 exhibit normal galactose-induct;ion kinetics for the chromosomal MEL1 gene expression and normal, albeit slower, log-phase growth. Insertion of the MEL1 gene into pMEGA2 provides a 24- to 30-fold increase in the Mel1 protein. Cells carrying a 2-mu m-based URA3-selectable plasmid containing a GAL1pro:lacZ reporter gene and a second plasmid, pMEGA2-Delta URA3, produce 12-fold more P-galactosidase than cells carrying only the GAL1pro:lacZ reporter plasmid. The performance of the MEGA plasmids in providing amplified production of the Gal3, Gal80, and Gal4 proteins should prove useful in investigations of the mechanistic aspects of these transcription switch proteins and in work aimed at achieving high-level, galactose-regulatable production of proteins in yeast.