In this article the effect of light quality on the size of the photosystem II (PSII) light harvesting complex (LHCII) is studied by measuring the chlorophyll fluorescence emitted by leaf sections of oat ("Avena sativa," var. Prevision) plants previously treated with either white light or with light filtered through blue, green, red or farred optical filters. Illumination with white light decreased the maximum fluorescence emitted by the leaf sections with respect to that emitted by sections adapted to darkness, which could indicate a loss of protein-pigment complexes bound to PSII. Treatment with green or blue light did not modify the maximum fluorescence emitted by the leaf sections, while illumination with red light caused a diminution in the maximum fluorescence emitted similar to that caused by white light. This effect of white and red light can be reversed by further illumination with far-red light. This article is intended for advanced students of plant biochemistry and plant physiology. (Contains 4 figures.)