Ripe and unripe Carica papaya fruits (epicarp, endocarp, seeds and leaves) were extracted separately and purified. All the extracts except that of leaves produced very significant antibacterial activity on Staphylococcus aureus, Bacillus cereus, Escherichia coli, Pseudomonas aeruginosa and Shigella flexneri. The MIC of the substance was small (0.2-0.3 mg/ml) for gram-positive bacteria and large (1.5-4 mg/ml) for gram-negative bacteria. The substance was bactericidal and showed properties of a protein. Other proteins previously found in C. papaya did not show antibacterial activity.