Leukocytes emit weak light in the visible region after the treatment with various opsonized particular or soluble stimulants. O-⋅2production is, kinetically, closely related to luminescence intensity. Luminescence obtained from the opsonaized zymosan-induced system with and wthout added bovine serum albumin has relatively intense peaks at or near 490, 530, 570, and 610-630 nm. Such spectra patterns in the region of 530-630 nm seem similar to those reported for the luminescence of photoproducts formed from indole analogs, assuming a red sift by solvent effect. Luminescence at 490 nm is also involved in an excited indole compound. A possible mechanism of emitter from leukocytes is discussed. The production of 1O2 if, it is generated, would be in a minimum amount at physiological pH. Furthermore, because of very short lifetime and high reactivity of 1O2 with O-⋅2, low physiological significance of 1O2 would be considered.