Serine hydroxymethyltransferase (SHMT) in the form of crude extract from a recombinant strain of Klebsiella aerogenes was used for the production of L-serine from glycine and formaldehyde (HCHO). A stirred tank bioreactor with a continuous feed of HCHO (37%) was employed. Since the performance of the serine bioreactor was heavily dependent on how HCHO was fed, an automatic feedback control system was developed for HCHO delivery utilizing the phenomenon of formol titration. This control procedure was based on the following circumstance: as a bioconversion proceeded, if the rate of HCHO feed was balanced by the rate of serine synthesis so that HCHO concentration was maintained near zero, then there was no pH change in the bioreactor. Once the rate of HCHO addition exceeded that of serine synthesis, the HCHO concentration built up and the excess HCHO reacted with the amino group of an amino acid (e.g. glycine or serine) to produce a Schiff base and a proton which lowered the pH. A pH controller detected and relayed this pH change to the on-off switch of the HCHO feed pump. Thus, HCHO infusion stopped when the pH was lower than the set point, which was the initial pH of the reaction. With this control system, the maximum concentration of HCHO that was reached in the bioreactor was only 1 mM-3.3mM depending on the pH and amino acid composition in the bioreactor. Moreover, a decrease in pH also signaled the use of a slower feed rate at which HCHO was to be delivered once the pH resumed its initial value after excess HCHO was consumed by the reaction. Employing this control system, we have optimized the performance of the serine bioreactor to give a serine titer of 450 g/L with an 88% molar conversion of glycine at a volumetric serine productivity of 8.9 giUh.