When the heterologous proteins thaumatin and bovine prochymosin are produced in yeast cells as a fusion with the yeast invertase secretory signal peptide, less than 2% of the product is secreted in a biologically active form into the medium. The remainder accumulates intracellularly in a misfolded conformation. We investigated whether this poor secretion can be improved by overexpression of binding protein (BiP) one of the major chaperones in eukaryotic cells. Indeed, a tenfold increase in the level of binding protein, as a result of the introduction of extra copies of the kar2 gene into yeast cells containing a single, integrated copy of the invertase/prochymosin fusion gene, caused more than a 20-fold increase in the amount of extracellular prochymosin. By additional disruption of the PMR1 gene of these cells we were able to obtain secretion of virtually all of the prochymosin produced. Export of thaumatin, on the other hand, was not significantly stimulated by binding protein overexpression.