The majority of the investigations were performed with the marine sponge Tethya lyncurium at concentrations of 2 X 10(-8) to 1 X 10(-11) g/ml of benzo[a]pyrene (BaP). Sea-pollution was characterized as BaP equivalent activity in the Ames test. Increased activity of ornithine decarboxylase (ODC) was observed when sponges were artificially exposed at polluted marine areas for 3 weeks. In contrast to the situation in higher animals no ODC induction of the fast type was observed. Mixed function oxygenases (MFO) were not detected in sponges nor could they be induced as in vertebrates. BaP was absorbed by Tethya and concentrated 30--60-fold. In live, but not dead, artificially perfused sponges [3H]- and [14C]BaP-radiolabeled became firmly associated with DNA, RNA and protein of the sponges. The association persisted in isolated fractions, in nucleotides, in nucleosides and in protein hydrolysates. The BaP binding ratio to DNA was found to be strongly correlated to the concentration of BaP. Light modifies BaP and thus enables binding. In the dark only very low association, if any, is observed. The possible consequences of these findings are discussed.