In chlorophyll-free, mutated Chlorella cells the respiratory oxygen consumption is enhanced upon irradiation with blue light. Since for other blue light-dependent reactions in plants modifying effects of red light have been reported, we tested the blue light-enhanced oxygen uptake of Chlorella after preceding, or during simultaneous, application of red light of different wavelengths and intensities. There was no measurable alteration. Also prior exposure to blue light did not create any measurable sensitivity to red light compared with darkness. The data obtained give no indication of an involvement of a red light absorbing pigment (e.g. phytochrome) in the respiration of Chlorella.