Kinetic formaldehyde study was carried out of the secondary DNA structure of calf goiter gland after in vitro irradiation with gelium-neon, gelium-cadmium, nitrogen and eximeric (on galogenides of inert gases) lasers. It has been shown that the number of defects in the secondary DNA structure depends on physical parameters of the lasers (first of all on generation wave length), and on the medium composition as well.