Experiments were conducted under growth chamber conditions in order to investigate the importance of the background light quality for the effect of UVB (280–320 nm) radiation on plants. Three week-old sugar beet (Beta vulgaris L.) plants were irradiated for three weeks with enhanced levels of UVB radiation with the following background light conditions: white light (WL) (350–700 nm), WL+UVA (supplementary UVA, 320–400 nm) and "yellow" light (YL) (450–700 nm). The effects of the enhanced UVB radiation were estimated on leaf area, fresh and dry weights of storage roots, chlorophyll and carotenoids, peroxidase activity, fluorescence induction and ultraweak luminescence (UL). Plants grown under YL+UVB died after 10 days. The lowest values for leaf area and fresh and dry weights of storage roots were observed under WL+UVA+UVB conditions. Under WL+UVA and WL+UVA+UVB the decrease in total chlorophyll was mainly a result of the reduction in chlorophyll a. Photosystem II appeared to be inhibited by UVB radiation, while the addition of UVA suggested an ameliorating effect with respect to the rise time kinetics of the chlorophyll fluorescence induction curves. UL of leaves was highest in plants grown under WL+UVB conditions. It is suggested that part of the UL derives from lipid peroxidation. Protective measures may have been afforded by the increase in carotenoids under YL and WL+UVB radiation. Also the leaf peroxidase activity was highest under WL+UVB conditions.