Specific, complete and reversible inhibition of the joining of blunt ended DNA duplexes catalyzed by the T4 DNA ligase can be obtained by using ATP, the enzyme cofactor, at concentrations of 5 mM and higher. On cohesive DNA ends, 5 mM ATP, which completely inhibits blunt end ligation, brings about only a limited (8%) reduction in the level of joining obtainable under optimal ATP concentra tion (0,5 mM or lower). A similar but less drastic uncoupling of the two kinds of joining can b$+achieved at lower ATP con_ centration(2,5mM)using1mMMg . The joining of DNA blunt ends can also be inhibited almost completely by 10 mM spermidine, without noticeable effect on the joining of cohesive termini.