Kinetics of inhibition of the growth of Acetobacterium sp. by methanol (substrate) and acetate (inhibitory end-product) were analyzed using a second-order substrate inhibition model coupled with a non-competitive product inhibition model. The analysis revealed that the substrate saturation constant K-s was 83.1 mM methanol; the substrate inhibition constant K-i was 277.8 mM methanol; the inhibition constant K-p was 0.835 mM undissociated acetic acid (239 mM of total acetic acid at pH 7.2); the exponent of inhibition n was 0.72; and mu(max) was 0.175 h(-1). The growth characteristics of Acetobacterium sp. in a defined medium containing methanol and CO2 were investigated and this medium was modified in order to optimize growth of the bacterium and production of vitamin B-12 in batch culture. The composition of the modified medium was as follows (per liter of deionized water) : methanol, 8 g; NaHCO3, 15.0 g; KH2PO4, 0.685 g; NH4Cl, 1.0 g; MgSO4 . 7H(2)O, 0.1 g; CoCl2 . 6H(2)O, 0.02 g; 5,6-dimethylbenzimidazole, 0.02 g; yeast extract, 2.0 g; L-cysteine-HCl . H2O, 0.5 g; trace element solution, 80 ml; vitamin solution, 40 ml; titanium (III) citrate, 0.015 mM. The maximum cell concentration of Acetobacterium sp. doubled (1.3 g/l) when the acetogen was grown in the modified medium compared with that in the basal medium (0.63 g/l).