A new antibiotic having .beta.-lactamase inhibitory activity, PA-39504-X.sub.3 of the formula: ##STR1## and the pharmaceutically acceptable slats being useful as a drug, a animal drug and a disinfectant for inhibiting the growth of gram-positive and gram-negative pathogenic microorganisms and a process for preparing the same, being characterized by cultivating Streptomyces argenteolus PA-39504 or Streptomyces tokunonensis PA-31088 in a suitable medium and isolating PA-39504-X.sub.3 from the cultured broth.