We observed that oyster-shell calcium treated electrically with ohmic heating delayed the spoilage of some foods. We carried out anti-microbial tests and application tests in foods. The calcium preparation and its major component, calcium hydroxide, showed almost the same minimum inhibitory concentrations (MICs) in the range of 0.07 to 0.1%, and no significant difference was observed between Gram-positive and negative bacteria. In the case of sodium and potassium hydroxide tested for comparison, MICs ranged from 0.09 to 0.15% and from 0.15 to 0.24%, respectively. Yeasts were inhibited at lower concentrations than bacteria. Viable bacterial cell number in food decreased by the addition of the calcium preparation. The growth of Bacillus, Escherichia, Salomonella and Staphylococcus inoculated to food was suppressed by the calcium preparation. Although this phenomenon was observed also for calcium hydroxide, the latter compound was not as effective as oyster-shell calcium. It was shown that the calcium preparation, used as a nutritional supplement could be applied to control the growth of microorganisms in food. Also, it was suggested that the preparation could be used in the alkaline range to keep the quality of food.