Growth, lactose utilization and S-adenosyl-L-methionine (AdoMet) production by Kluyveromyces lactis AM-65 on whey in batch fermentation were investigated and an unstructured model of the process has been derived. The optimal set of parameters was estimated by fitting the model to experimental results. After incubation for 20 h the optimal fermentation conditions (28.5degreesC, pH 5.3, agitation at 270 rpm) resulted in AdoMet production at 1.55 g l(-1).