Histidine decarboxylase production from Lactobacillus hilgardii 5w, isolated from wine, was inhibited by the presence of L-malic acid in the basal culture medium. The inhibition was related to L-malic acid concentration. The maximal production of the enzyme at 12 h of culture incubated at 30 degrees C was inhibited 71% by 2 g/L L-malic acid and 47% by 0.5 g/L. In these conditions L-malic acid consumption was 16% and 20% respectively. The addition of 300 mg/L citric acid to the basal medium stimulated the enzyme production from 9 to 45 nmoles/min/mg dry weight, and the increase was correlated with citric acid concentration. When different concentrations of L-malic acid were added to the basal medium plus 200 mg/L citric acid, reversion of stimulation was observed, achieving the maximum at a concentration of 2 g/L. In this case, citric acid comsumption was not modified, whereas L-malic acid utilization was higher.