The action spectrum of apparent photosynthesis for attached radish (Raphanus sativus L. var. Early Scarlet Globe) and corn (Zea mays L. var. Pride V.) leaves was measured at 300 mul/l CO(2) and both 21% and 2% O(2). The spectra were measured at light intensities where apparent photosynthesis was proportional to intensity. For radish, a high compensation point plant, oxygen had an inhibiting effect on photosynthesis at all wavelengths from 402 to 694 mmu. If a constant rate of photosynthesis at 21% O(2) for the different wavelengths was chosen, then the percent increase in net CO(2) fixation at 2% O(2) was constant. For corn, a low compensation point plant, no inhibitory effect of oxygen concentration from 2% to 21% O(2) was found over the visible spectrum. The CO(2) compensation point for light intensities greater than the light compensation point was found to be constant and independent of wavelength for both radish and corn leaves. For radish, the lowering of the oxygen concentration from 21% to 2% at these intensities was found to reduce the CO(2) compensation point by the same amount for the wavelengths studied.