Lactobacillus acidophilus AC1 was found to produce a proteinic inhibitor having a molecular weight of 5.4 kd, active against both Gram-positive and Gram-negative bacteria. Some of the sensitive bacteria were found to be resistant to most of the commonly employed drugs and antibiotics. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of the purified inhibitor showed the presence of a single polypeptide. The inhibitor was active over a wide pH range from 4.0 to 7.5. It was heat sensitive, and complete inactivation occurred within 20 min at 50 degrees C. The purified inhibitor lost 50% of its activity within 24 h at room temperature and after 5 days at 4 degrees-8 degrees C. The inhibitory protein was readily distinguished from other inhibitors in lactic cultures by its immunological cross-reactivity.