Dependences of nonlinear laser cleavage of DNA on a concentration of t
he free-radical scavenger, beta-mercaptoethanol, have been measured. T
he obtained data rule out the free-radical mechanism of the nonlinear
laser cleavage and agree with the mechanism based on two-quantum excit
ation energy transfer from a dye to DNA. A high concentration of beta-
mercaptoethanol has been found to cause DNA hyperchromicity.