A system for the controlled expression of a foreign gene in Saccharomyces cerevisiae by temperature and/or inorganic phosphate (P-i) concentration in the medium was constructed. A DNA fragment bearing the promoter of the PHO84 gene, which encodes a P-i transporter of S. cerevisiae and is derepressed by P-i starvation, was used as promoter. When a cDNA fragment encoding the human lysozyme (h-lysozyme) gene connected with the PHO84 promoter was ligated into a YEp vector, a maximum of 4.5 mg/l of the enzyme was secreted from the host cells in low-P-i medium. When a temperature-sensitive pho81 mutant was used as the host with this vector, 2.6 mg/l of h-lysozyme was secreted in low-P-i medium at 25 degrees C and its production was turned off at 37 degrees C.