We surveyed the processes of weakening of light in biological mediums. We described the procedure of definition of an extinction coefficient, and also scoring of erythrocytes. Also we experimentally obtained values of coefficient in dependence on a wavelength of radiance and from concentration of scattering centers. We detected influencing erythrocytes on weakening of a laser radiation at transit through a layer of a blood. We constructed the graphs of an extinction coefficient against quantity of red cells in one microliter of a blood.