– Cyclo(L-Leu-L-Trp) is a diketopiperazine metabolite initially derived from Penicillium, exhibiting antibacterial (MICs = 125-1000 µg/ml) and antifungal activities (MICs = 8-64 µg/ml). It also demonstrates antioxidant properties by reducing the production of hydroxy radicals, as evidenced in an electron spin resonance (ESR) spectroscopy assay (IC50 = 1.8 µM). Additionally, Cyclo(L-Leu-L-Trp) has been identified as a bitter compound capable of quickly crossing rat taste cell membranes ex vivo at a 1 mM concentration. Furthermore, it acts as a melatonin receptor agonist in X. laevis melanophores, suppressing cAMP accumulation at a 20 µM concentration, with this effect being negated by the melatonin receptor antagonist luzindole.