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In vitro experiments were conducted with trypsin and Na-Benzoyl-l-arginine-p-nitroanilid (L-BAPA) as substrate. The aim of the work was to explore the influence of additional Cu2+, Zn2+, Co2+, Ni2+ and Fe3+ ions in the reaction mixtures on the trypsin activity at different enzyme-substrate ratios (E : S = 1 : 100; 1 : 50; 1 : 10; 1 : 5). The reaction conditions were chosen according to the steady-state conditions of the tryptic hydrolysis. The trypsin activity increased by adding small amounts (4.5 X 10(-7) mol Cu2+/l) and decreased by adding higher amounts of Cu2+ ions. There was no influence of the E : S ratio. The tryptic activity was only partly influenced by the substrate concentration when the other trace elements were added to the reaction mixtures. These results may explain in part the improved protein digestibility which is reported when 250 mg Cu/kg are added to pig feeds.