Continuous fermentation was applied to the production of recombinant human chymotrypsinogen B (hCTRB) by the methylotrophic yeast Pichia pastoris as a tool for the kinetic analysis of growth and product formation. Using methanol as the sole source of carbon, energy, and induction, cell growth could be described by a non-competitive M ON approach. The maximum growth rate p a,, was determined to be 0.084 h(-1) and the K-M-value for methanol to 0.22 g L-1, respectively. With respect to product formation a similar model was established exhibiting a methanol concentration of 0.13 g L-1 as the K-M-value and a maximum biomass-specific product-formation rate Of pi(max) = 0.23 mg g(-1) h(-1). The production of hCTRB was strictly growth-coupled. The data provided covers the range of methanol concentrations between 0 and 4 g L-1. Substrate concentrations exceeding this upper value led to a complete collapse of product formation. This change in phenotype turned out to be irreversible indicating a genetic instability of transformed Pichia pastoris caused by excess methanol.