Production of recombinant human serum albumin (rHSA) controlled by the constitutive promoter phosphoglycerate kinase was studied in Kluyveromyces lactis. It was governed by both cell concentration and glycolytic flow. The triggering of the fermentation metabolism by unfavourable culture conditions (pH, pO2, D) caused a decrease in the synthesis of the heterologous protein. The highest productivity (75 mg l-1 per h) and rHSA concentration (62 mg l-1) were obtained in chemostat culture with a dilution rate of 0.12 h-1 and with 38 g l-1 dry weight.