Acute injury threshold lelvel MRD50 of CO2 laser light for the skin of eight white piggies was measured. On the basis of the animal experiment, the same measurement for the skin of six yellow people, was made. MED50 was found to be 2.3 J/cm2. The skin of 5 volunteers was exposed to the above energy densities,and histologieal examination found that capillaries are expanded and filled with erythrooytes and light edema, but there is no significant change of cuticle. This indicates that the MBD50 erythematous reaction is minimal and reversible.