An aqueous extract of freeze-dried garlic (Allium sativum), when incorporated into growth media, inhibited many representative bacteria, yeasts, fungi and a virus. All microorganisms tested were susceptible to garlic. Quantitative assessment of the minimum inhibitory concentrations for bacteria and yeasts showed values ranging from 0.8 to 40.0 mg garlic ml(-1). Fungal radial colony growth was inhibited by at least 25% at concentrations as low as 2.0 mg garlic ml(-1). The 50% endpoint neutralization titre for rotavirus was 2.4 to 2.8 μg ml(-1). Lactic acid bacteria were the least sensitive microorganisms to the inhibitory effects of garlic. In mixed culture studies of Lactobacillus acidophilus and Escherichia coli, garlic prevented the establishment of E. coli, although the final outcome of competition was not affected.