Efficient secretion of human lysozyme from the yeast, Kluyveromyces lactis, was achieved by using more stable vectors in the order of S11 replication origin-containing episomal vector < full-length K. lactis plasmid pKDI-containing vector < centromeric vector < chromosome-integrated vectors. Cells containing a PGK (phosphoglycerate kinase) promoter-driven integration vector grown in non-selective rich medium achieved the highest level of secretion, similar to100 mug lysozyme secretion ml(-1) culture: this level was similar to10-fold higher than that achieved by episomal vectors. An additional copy of the protein disulfide isomerase gene further facilitated the secretion.