Recently we reported the isolation and structure determination of three ichthyotoxic, bisindole alkaloids, topsentins A, B1, and B2, from the Mediterranean sponge Topmrza gmitrix Schmidt (Epipolaidae) (1). A further examination of the most polar fraction of the MeOH extract of this sponge led to the isolation of 3-methyladenine identified on the basis of its spectral properties. This identification was further proved by direct comparison with an authentic sample prepared by methylation ofadenine with Me1 in DMF (2).A few free purine derivatives (3-5) and some glycosylpurines (6-9) already have been isolated from sponges. Moreover, several diterpenic purine derivatives have been reported from Agelas species (10-15).As a free base, 3-methyladenine has never been reported from natural sources, but 3-methyladenin.e-DNA and 3-methyladenine-RNA are formed when cells or cell extracts are treated with methylating agents. These apurinic sites may be selectively incised by an endonuclease with subsequent excision repair by the action of specific enzymes (16).