Penicillic acid is a toxic metabolite of various Aspergillus spp., Penicillium spp. and a marine-derived Exophiala sp. discovered in the early 1900s. Its structure was reported in 1936 by Raistrick and co-workers. Penicillic acid exists as a tautomeric butenolide in solution. Penicillic acid has broad biological activity, as an antimumor, antiviral, antibacterial and quorum sensing active. Penicillic acid is a potent inhibitor of alginate biosynthesis with MIC of 6 μg/mL for 86% inhibition of alginate production. Penicillic acid also inhibits LPS-induced NO production and NF-κB activation.