The antitumor drug 4'-(9-acridinylamino)methanesulfon-m-anisidide which stimulates the cleavable complex formation between mammalian DNA topoisomerase II and DNA also stimulates the cleavable complex formation between bacteriophage T4-induced DNA topoisomerase and DNA. In the presence of 4'-(9-acridinylamino)methanesulfon-m-anisidide, T4 DNA topoisomerase and DNA form a "cleavable complex" which is characterized by its sensitivity to protein-denaturant treatment. Upon protein-denaturant treatment, the phosphodiester bond of DNA is cleaved, and the gene 52 protein subunit of the topoisomerase becomes covalently linked to the 5'-end of the broken DNA. The covalent protein-DNA linkage has been determined by both paper electrophoresis and thin layer chromatography to be tyrosyl phosphate.