Expression of the human interferon-beta (hIFN-beta) gene was found to be very toxic for Saccharomyces cerevisiae. An integrative expression cassette, containing the hIFN-beta gene under control of the inducible galactokinase (GAL1) promoter in combination with the alpha-factor prepro-secretion signal, was used to study the secretion process in more detail. Specific differences were found between a vacuolar proteinase--mutant and a normal laboratory yeast strain. Cell organelle fractionation, carried out with the recombinant C13-ABYS66 strain, revealed that 99% of the hIFN-beta remained intracellular and that the majority was associated with the vacuolar fraction. The secretion efficiency in the latter strain was investigated by overexpressing chaperone molecules (HSP70 and BiP) and homologous secretion factors (SEC1 and SEC18). Only the presence of HSP70 resulted in a 5-fold increase in secreted hIFN-beta.