The stimulatory or inhibitory effects of plant lectins on the production of gelatinase A (MMP-2) and gelatinase B (MMP-9) by mononuclear white blood cells was investigated by substrate zymography. Leukocyte cultures from 24-h old buffy coats were spontaneously activated and produced high levels of gelatinase B. Using such cultures the suppressing activity of the Datura stramonium, Viscum album, Bauhinia purpurea, Triticum aestivum and Maackia amurensis lectins on gelatinase B induction were demonstrated. When fresh leukocyte preparations from single blood donors were used, low levels of gelatinase B were produced. The induction of gelatinase B was confirmed for concanavalin A and phytohaemagglutinin (PHA-L4). In addition, the Urtica dioica, Calystegia sepium, Convolvulus arvensis and Colchicum autumnale lectins were documented as novel and potent inducers of gelatinase B. Since high circulating gelatinase B levels are associated with specific pathologies, including shock syndromes, the acute toxicity of many lectins might be partially mediated or influenced by gelatinase induction.