[en] Polyamines, putrescine, spermidine, and spermine are organic cations present in all cells. They have multiple regulatory roles in cell growth and differentiation. 2,3,7,8-Tetrachlorodigenzo-p-dioxin (TCDD), and related polycyclic aromatic hydrocarbons exert their adverse effects by binding to an intracellular protein (Ah receptor). Interaction of Ah receptor with specific DNA sequences triggers gene regulatory effects of TCDD. The authors examined the effect of polyamines on the structure and DNA binding of Ah receptor isolated from a human squamous carcinoma cell line, A431. [³H]TCDD-labeled Hf receptor was sedimented in the 9S region of sucrose gradients in hypotenic buffer. Polyamines caused a concentration dependent condensation and precipitation of Ah receptor. In the presence of 1 mM spermine the receptor was completely precipitated which could be recovered from the bottom of the sucrose gradients. This precipitation did not occur with RNase-treated Ah receptor. Incubation of RNase-treated Ah receptor with 1 mM spermidine increased its DNA binding 10-fold compared to controls having equivalent ionic strength. These results suggest that endogenous polyamines may influence the structural organization and gene regulatory effects of TCDD