New types of antimicrobial compounds were identified in the culture filtrate of Lactobacillus plantarum VTT E‐78076. Activity was detected in the low molecular mass fraction separated by gel chromatography. This fraction totally inhibited the growth of the Gram‐negative test organism, Pantoea agglomerans (Enterobacter agglomerans) VTT E‐90396. Characteristic compounds from this fraction were identified by GC/MS‐analysis and the identification was confirmed using pure commercial reference compounds in identical chromatographs and in antimicrobial tests. The active fraction included benzoic acid (CAS 65–85–0), 5‐methyl‐2,4‐imidazolidinedione (CAS 616–03–5, methylhydantoin), tetrahydro‐4‐hydroxy‐4‐methyl‐2H‐pyran‐2‐one (CAS 674–26–0, mevalonolactone) and 3‐(2‐methylpropyl)‐2,5‐piperazinedione (CAS 5845–67–0, cyclo(glycyl‐l‐leucyl)). These compounds in concentrations of 10 ppm inhibited growth of the test organism by 10–15% when acting separately, but 100% when all were applied together with 1% lactic acid. The inhibition was 40% by 1% lactic acid alone. The compounds were also active against Fusarium avenaceum (Gibberella avenacea) VTT D‐80147. The inhibition was 10–15% by separate compounds in concentrations of 10 ppm and maximally 20% in combinations. Fungal growth was not inhibited by lactic acid. Inhibition by unfractionated Lact. plantarum culture filtrate was 37% and by the low molecular mass fraction, 27%.