Monoclonal antibodies were used in an enzyme-linked immunosorbent assay (ELISA) to detect the induction and removal of cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA isolated from ultraviolet B (UV-B)-exposed primary wheat (Triticum aestivum L. cv. Chinese Spring) leaf tissue. The accumulation of lesions in the primary leaves of 6-d-old wheat seedlings was followed during the exposure of the leaf to an approximate dose of 3.6×10−1 W m−2 UV-B (Caldwell weighting). Significant increases in the levels of both CPDs and (6-4) photoproducts were detected in wheat leaves exposed to UV-B in the absence of other light However, only an increase in (6-4) photoproduct levels could be measured in wheat leaves exposed to the same UV-B source in the presence of supplemental white light. The removal of CPD antibody binding sites in the DNA after irradiation was rapid under conditions of high light intensity in contrast to the removal of (6-4) photoproduct antibody binding sites, which was significantly slower. The removal of CPDs appeared to be light dependent, this rate of removal decreasing with decreasing light fluences. The removal of (6-4) photoproducts also appeared light dependent, but to a lesser extent than the removal of CPDs, under the conditions studied here. Gene expression in the primary wheat leaf was measured and showed an up-regulation of chalcone synthase expression and a reduction in expression of chlorophyll a/b-binding protein (cab) in response to supplementary UV-B. No effect was seen on the expression of the other photosynthetic genes studied (the genes coding for the enzymes sedoheptu-lose 1,7-bisphosphatase and fructose 1,6-bisphosphatase). Measurement of the levels of DNA lesions in this same tissue showed that the observed changes in gene expression accompanied the appearance of UV-B induced lesions in the form of (6-4) photoproducts in the wheat leaf genome.