The screening of soil actinomycetes for β-lactamase inhibitors is described. Using a plate test a number of strains of Streptomyces were found to produce β-lactamase inhibitory activity designated olivanic acid complex. Factors affecting the production of this complex by Streptomyces olivaceus ATCC 21379 are reported. The complex showed antibacterial activity and also inhibited a number of different types of β-lactamase in a progressive manner. Certain ampicillin-resistant bacteria are rendered sensitive to ampicillin in the presence of olivanic acid complex at a concentration which alone did not inhibit growth.