Drug interaction of a new antibiotic, methampicillin lysinate (MAL) with nine drugs were investigated using four species of gram positive and gram negative bacteria. The experimental results were as follows: 1. MIC of MAL were found to be decreased against E. coil when combined with mefenamic acid, probenecid, aluminium hydroxide gel or corticosteroids. The other drugs did not affect MIC of MAL against the same bacteria. 2. MIC of MAL were found to be increased against Staphylococcus aureus ATCC 6538-P, 9441 when combined with mefenamic acid, aluminum hydroxide gel or dexamethasone acetate. The other drugs did not affect MIC of MAL against the same bacteria. 3. MIC of MAL were found to be increased against Shigella dysenteriae when either of the nine drugs was combined. 4. MIC of MAL were found to be increased approximately 2.5 times when combined with Streptokinase-Streptodornase or hydrocortisone and to be decreased approximately 2 times when combined with probenecid or dexamethasone against Salmonella typhi(type 2). It seems the other drugs do not affect the MIC of MAL against the same bacteria.