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Epidemiology and physiological consequences of chlamydial infections show these intracellular bacteria to have maintained their prevalence. Especially C. pneumoniae are able to confer to a treatment refractory chronic state of infection that cannot be eradicated with currently available therapeutic options. Here we report the effects of biochanin A on the growth of intracellular Chlamydia spp. It is the main flavonoid component of red clover (Trifolium pratense) extracts, which besides its estrogenic and antioxidative properties is known to potentiate the antibacterial effects of other chemical agents by inhibiting bacterial efflux pumps. We identified biochanin A as a hit compound in a high-content screen of purified natural products for C. trachomatis growth inhibitors. It was found to inhibit the replication of C. pneumoniae clinical strain K7 (IC =12μM) and to prevent 100% of infectious progeny production at 50μM. Thus, biochanin A is a more potent inhibitor of C. pneumoniae  than the related isoflavone genistein, which we have earlier shown to be only moderately active against this bacterium. Further, this data suggests that biochanin A acts as a direct growth inhibitor rather than an antibacterial potentiator against these pathogens.