Field experiments assessing UV-B effects on plants have been conducted using two contrasting techniques: supplementation of solar UV-B with radiation from fluorescent UV lamps and the exclusion of solar UV-B with filters. We compared these two approaches by growing lettuce and oat simultaneously under three conditions: UV-B exclusion, near-ambient UV-B (control) and UV-B supplementation (simulating a 30% ozone depletion). This permitted computation of “solar UV-B” and “supplemental UV-B” effects. Microclimate and photosynthetically active radiation were the same under the two treatments and the control. Excluding UV-B changed total UV-B radiation more than did supplementing UV-B, but the UV-B supplementation contained more “biologically effective” shortwave radiation. For oat, solar UV-B had a greater effect than supplemental UV-B on main shoot leaf area and main shoot mass, but supplemental UV-B had a greater effect on leaf and tiller number and UV-B–absorbing compounds. For lettuce, growth and stomatal density generally responded similarly to both solar UV-B and supplemented UV-B radiation, but UV-absorbing compounds responded more to supplemental UV-B, as in oat. Because of the marked spectral differences between the techniques, experiments using UV-B exclusion are most suited to assessing effects of present-day UV-B radiation, whereas UV-B supplementation experiments are most appropriate for addressing the ozone depletion issue.