Yeast expression plasmids containing an appropriate leader sequence and bovine $eta$-casein cDNA were constructed to produce $eta$-casein for the study of its functional characteristics. Two kinds of expression systems for $eta$-casein were constructed using pCGYl444 as a precursor plasmid. This plasmid is a yeast-E. coli shuttle vector which contains the chelatin promoter. The plasmid pISB202 contains the invertase leader sequence and $eta$-casein gene. The plasmid pDEB303 contains the original bovine $eta$-casein leader sequence gene. These two plasmids were introduced into S. cerevisiae AB116 which is a strain deficient in the major yeast proteinases. Each clone was grown in minimal media for 24 h before induction by $CuSO_4$. The cells were thus grown under expression conditions. Both strains harbouring pISB202 and pDEB303 expressed bovine $eta$-casein. The $eta$-casein was detected using immunochemical staining after western blot. Secretion of $eta$-casein was detected in the culture broth. The estimated amount of secreted $eta$-casein was approximately 50 ${MU}g$/l.