Humans are permanently exposed to a variety of exogenous substances via air, food, and direct contact. Water soluble, hydrophilic, substances can readily be excreted in most instances while lipophilic molecules tend to accumulate in the body and may potentially cause adverse effects. Two groups of widely dispersed environmental pollutants causing a global contamination are polycyclic aromatic hydrocarbons (PAH) and persistent halogenated aromatic hydrocarbons (HAH) including dioxins and dioxin like chemicals. Although being different in several respects these chemicals have one property in common; their toxicity is strongly associated with the aryl hydrocarbon receptor (AHR) signaling pathway. While this connection is well established the molecular mode of action is still not fully revealed, since the long sought for endogenous AHR ligand was not identified. We have identified an endogenous ligand, 6-formylindolo[3,2-b]carbazole (FICZ), and can therefore use this tool to study the physiological role of the receptor and determine how toxic agents interfere with this basic signaling pathway. The research is done in collaboration with Agneta Rannug’s research group at IMM, KI.