More than 15 bacteriocins of Lactobacillus plantarum have been described. However, little information has been published on the fermentation optimization of these peptides. Plantaricin 423, produced by L. plantarum 423, is produced during exponential growth and reaches a maximum activity [6000 activity units (AU)/ml] at the beginning of stationary growth (i.e. after 15 h), This activity level is maintained for 3 h, but declines to 2600 AU/ml towards the end of fermentation (i.e. after 31 h). The increase in plantaricin 423 activity coincided with a decrease in pH from 5.8 to 4.0 during the first 15 h of fermentation. However, when the number of cells are taken into account (ODmax-value), a higher concentration of plantaricin 423 is produced in medium with an initial pH of 4.90 (2961 AU/ml/ODmax) than at pH 5.80 (2368 AU/ml/ODmax). A much lower activity of plantaricin 423 was obtained during the same fermentation period (15 h) when cells were grown in MRS broth (Merck) with an initial pH of 6.9. The production of plantaricin 423 increased from 6000 AU/ml in MRS broth (Merck) to 9600, 12,800 and 19,200 AU/ml when the medium was supplemented with 1.9% (w/v) meat extract (Oxoid), 3.4% (w/v) casaminoacids (Oxoid) or 1.9% (w/v) tryptone (Oxoid), and 1.7% (w/v) bacteriological peptone (Oxoid), respectively, The activity of plantaricin 423 was even further increased with the addition of 1% (w/v) tween 80 to MRS broth (Merck). Low concentrations of MnSO4. H2O (0.014%, w/v) stimulated the growth of strain 423 and increased the activity of plantaricin 423. Although the addition of MgSO4. 7H(2)O had the same stimulating effect on the growth of strain 423, the activity of plantaricin 423 was not increased.