The methylotrophic yeast Pichia pastoris is one of the best hosts for the production of foreign proteins because of the presence of the strong AOX1 promoter induced by methanol. Methanol feeding during the production phase of the foreign proteins is important because methanol not only induces protein production but also provides energy source for the host cells. Excess methanol inhibits the growth of host cells, while an insufficient amount of energy source and/or methanol starvation lead to poor growth and production. We constructed a simple methanol control system consisting of a semiconductor gas sensor and a relay. Using this system, we studied the effect of methanol concentration on the production of a model foreign protein, human beta(2)-glycoprotein I domain V. The methanol concentrations were kept constant at 1.5, 10, 17, or 31 g.l(-1) (+/-5%) during the production phase. Although the specific rates of growth and methanol consumption decreased with increase in the methanol concentration, the specific production rates increased, indicating that the energy for the production competed with that for cell growth. Accordingly, we provided glycerol as an extra energy source during the production phase, with the result that the specific production rate increased two times. Our simple and inexpensive system will help bioengineering studies on the production of recombinant proteins in P. pastoris, the growth and production of objective proteins in which are dependent on the methanol concentration.