Crude juices of garlic (Allium sativum), onion (Allium cepa) and shallots (Allium ascalonicum) were tested in an agar diffusion test for their growth inhibitory effect on five gram negative and three gram positive bacterial species and two yeast species. All test organisms were inhibited by garlic juice, whilst onion and shallot juice showed no effect upon gram negative bacteria. Garlic juice was investigated in more detail. Addition of complex-forming agents and organic matter to the crude juice reduced its activity on all test organisms. Volatile substances showed a strong inhibitory activity after exposure for 8 hours or longer at 23 degrees C or 37 degrees C. Minimal inhibition concentrations determined in a dilution test were found to be high for gram negative bacteria and low for both yeast species. The D-values of the different test organisms in undiluted garlic juice were calculated. P. aeruginosa had a very low D-value, whilst the bacteriostatic concentration was high. This indicates a large concentration exponent of crude garlic juice for this organism. The opposite was found for S. aureus. In view of the strong antibiotic properties and the complete absence of development of resistance further investigation upon the principles of the antimicrobial activity of juices from Allium species merits consideration.