Reaction of anthramycin 11-methyl ether (AME) with trifluoroacetic acid results in formation of (1,11a)-didehydroanhydroanthramycin (DAA). Anthramycin biosynthetically labelled from DL-[3'RS(3'-3H)]; DL-[3'S(3'-3H)] and DL-[3'R(3'-3H)] tyrosine each lose approximately 50% of their tritium during this conversion to DAA confirming the labelling pattern of 3'-tritiated species of tyrosine in AME. As expected negligible losses of tritium occurred from AME biosynthetically labelled fron L-[2- or 6-3H] or L-[3- or 5-3H]tyrosine. DAA did not form a stable adduct with DNA in accord with the postulated mechanism of action of anthramycin.