PIVALOYLOXYMETHYL 6 - (D -A - SULFOAMINOPHENYLACETAMIDO) PENICILLANATE SODIUM SALT (I) IS A VALUABLE ANTIBIOTIC AGENT, NUTRITIONAL SUPPLEMENT IN ANIMAL FEED, THERAPEUTIC AGENT IN POULTRY AND ANIMALS, INCLUDING MAN, AND IS ESPECIALLY USEFUL IN THE TREATMENT OF INFECTIOUS DISEASES CAUSED BY GRAM-POSITIVE AND GRAM-NEGATIVE BACTERIA, MOST PARTICULARLY THOSE CAUSED BY THE PSEUDOMONAS GENUS, SODIUM PIVALOYLOXYMETHYL 6-(D-A-SULFOAMINOPHENYLACETAMIDO)PENICILLANATE IS PREPARED BY THE TREATMENT OF DISODIUM 6-(D-A-SULFOAMINOPHENYLACETAMIDO) PENICILLANATE (II) WITH CHLOROMETHYL PIVALATE IN HEXAMETHYLPHOSPHORTRIAMIDE. COMPOUND I HAS A SIMILAR ANTIBACTERIAL SPECTRUM TO THAT OF COMPOUND II BUT IS ABSORBED FROM THE GASTRO-INTESTINAL TRACT 2-3 FOLD MARE EFFICIENTLY THAN COMPOUND II.