Photon emission (PE) from yeast cells Saccharomyces cerevisiae strain SP-4 in normal conditions and in conditions perturbed by the addition of formaldehyde was investigated using single-photon counting equipment. PE from yeast cells, growing in a standard nutrient medium (YPG) then centrifuged and resuspended in a phosphate buffer (pH = 6.5), was measured in the presence of oxygen or argon. The solution of formaldehyde (2%) was injected into the sample. The intensity of PE increased and reached a maximum, then slowly decreased to a level which was higher than the PE level without the perturbing factor. The kinetics of PE was found to be strongly dependent upon the presence of oxygen. The model of formation and recombination of free radicals was tested. The results indicate that PE can arise during the recombination reactions of free radicals like R. + R., RO. + RO., RO2. + RO2. which are formed in the enzymatic oxidative reactions.