Isopimaric acid is a diterpenoid resin acid that has been found in P. nigra and has diverse biological activities.^1,2,3,4 It is active against clinical isolates of epidemic methicillin-resistant S. aureus (EMRSA; MICs = 32-64 µg/ml).¹ Isopimaric acid is an agonist of retinoid X receptor alpha (RXRα), RXRβ, and RXRγ in a reporter assay using HEK293T cells expressing human receptors (EC50s = 26, 32, and 33 µM, respectively).² It opens large conductance calcium-activated potassium channels (BKCa1.1/KCa1.1) in HEK293 cells expressing recombinant channels and the activating β1 regulatory subunit when used at a concentration of 10 µM.³ Isopimaric acid potentiates reductions in field excitatory postsynaptic potential (fEPSP) slopes in hippocampal slices and decreases escape latency in the Morris water maze in a 3xTg mouse model of Alzheimer’s disease.⁴ It has been found as an environmental contaminant in pulp and paper mill effluent.⁵