Results were based on plant response with light levels in energy units, in accord with use of the spectrum by plants. Dry weight yields of control plants, and reductions of yields by 2,4-dichlorophenoxyacetic acid (2,4-D) below those of control plants were greatest in red light, followed by pink, yellow, white, green, and blue light, in that order. Sucrose mixed with 2,4-D or applied alone to plants altered the combined light-herbicide effect but little. Differences in concentration of 2,4-D affected respiration more than photosynthesis, as measured by CO2 exchange under different light qualities. In spectral effects, pink light caused greatest reduction of photosynthesis by 2,4-D at 500 ppm, while greatest promotion of respiration occurred under red light with 2,4-D at 1000 ppm.