The influence of carbon sources on rhGH (recombinant human growth hormone) production by two Pichia pastoris strains having different methanol utilization phenotypes (P. pastoris-hGH-Mut(+) and P. pastoris-hGH-Mut(s)) was investigated using batch bioreactors. The effect of methanol concentration (C(MeOH)) in defined and complex media, and further glycerol/methanol mixed defined media, was analysed systematically over a wide range. With methanol as the sole carbon source, strain Mut(s) grew only slightly, whereas with Mut(+), a cell concentration (C(X)) of 6.0 g of dry cells/dm(3) was obtained and an rhGH concentration (C(rhGH)) of 0.032 g/dm(3) was produced. In complex medium without glycerol at a C(MeOH) of 2% (v/v), a C(rhGH) of 0.16 g of rhGH/dm(3) was produced by Mut(s), a value 3-fold higher than that produced by Mut(+), despite the fact that the C(X) of Mut(+) (6.1 g/dm(3)) was 2-fold higher than that of Mut(s) (3.0 g/dm(3)). In a glycerol/methanol mixed defined medium, methanol consumption began when glycerol was totally depleted, indicating that glycerol is a repressor of the AOX1 (alcohol oxidase-1 gene) promoter. With strain Mut(s) at a glycerol concentration (C(Gly)) of 30 g/dm(3) and a C(MeOH) of 1% (v/v), the C(rhGH) produced was 0.11 g/dm(3), whereas, with the Mut(+) strain, a C(rhGH) of 0.06 g/dm(3) was obtained at a C(Gly) of 30 g/dm(3) and a C(MeOH) of 4%. As methanol is not consumed by Mut(s) strain effectively and the presence of methanol in the fermentation broth triggers induction of the AOX1 promoter, our results encourage the use of the Mut(s) strain for rhGH production. In addition to rhGH production, the specific cell growth rates, specific methanol and/or glycerol utilization rates and maintenance coefficients in methanol- and glycerol-based defined media were determined. With a methanol-based defined medium and using the Mut(+) strain, a higher specific growth rate (mu) of approx. 0.14 h(-1) was observed during the exponential cell growth phase at a C(MeOH) of <or=2.0%. When glycerol was used as a sole carbon source, both phenotypes showed similar cell-growth and glycerol-utilization rates. The results of the present study should enable one to optimize the expression of other therapeutic proteins by P. pastoris.