Estimates of viability as measured by vital staining with fluorescein diacetate (FDA) were carried out on freshly isolated and partially aged (16-h-old) B. vulgaris and H. vulgare mesophyll protoplasts following irradiation with UV-B. Damage to the photosynthetic system by UV-B was determined by delayed light emission (DLE). In the case of freshly isolated protoplasts Beta was .apprx. 30% more susceptible than Hordeum following 3 h irradiation, with viability decreasing from 90% to 40%. After storage of protoplasts on ice for 16 h UV-B radiation markedly depressed viability in both species, but in the case of Hordeum there was a substantial initial loss of nearly 70% in viability over the 1st hour of irradiation. The first 10 min of UV-B radiation decreased the intensity of DLE by 40% without appreciably affecting the decay rate. Longer treatment times did not give a proportional effect so that even after 60 min of UV-B the inhibition did not exceed 60%. This suggested that although the enzyme system responsible for FDA hydrolysis may be partially inactivated (viability was 75-80% as compared with 90% in the control), the UV-B did not penetrate the innermost parts of the chloroplasts, but left some thylakoids undamaged.