The response of UV-B radiation (UV-B-L; 0.5) and (UV-B-H; 1.5 mu mol m(-2) s(-1)) was investigated in two cyanobacteria-Nostoc muscorum and Phormidium foveolarum-preacclimated to low, normal, and high (LL; 15 +/- 0.5, NL; 75 +/- 3.5, and HL; 225 +/- 5.5 mu mol photon m(-2) s(-1), respectively) light intensities. Acclimation to HL stimulated growth over the control; however, both UV-B doses induced decline in growth of both cyanobacteria. UV-B-induced decline was maximum in LL acclimated cells which was in consonance with the results for chlorophyll content. LL acclimation led to an increase in phycocyanin content. Carotenoids content was maximum in HL acclimated (UV-B untreated and treated) cells of N. muscorum and P. foveolarum. Maximum stimulation in SOR and H2O2 levels was observed under LL + UV-B-H treatment which was correlated with inefficient antioxidative mechanism (SOD, CAT, POD, and GST) in these cells. MDA and RCG contents also showed a similar trend. Mitigation of UV-B-induced stress response could be established in HL acclimated cells due to the concerted and differential display of enzymatic and nonenzymatic antioxidants in them. P. foveolarum proved to be more resistant than N. muscorum under the studied test conditions.