The growth response of the marine alga Dunaliella tertiolecta to different concentrations of lead and aluminum was investigated. Both metals had a stimulatory effect at low concentration and an inhibitory effect at high concentration (hormesis). The IC25 values of lead are 8.43, 7.29, and 6.74 mg L-1 for 24, 48, and 72 h, respectively. The corresponding values for aluminum are 30.54, 22.42, and 18.16 mg L-1. Although it seems that the two metals are not directly toxic to the alga at the concentrations found in the environment, as implied by the IC25 values and the environmental concentrations of the metals, low concentrations of both metals, alone and in combination, affected the ultrastructure. The growth of batch-grown cells exposed to 0.5 mg L-1 lead and aluminum, alone and combined, during the 24-h exponential phase was investigated. The same cells were also examined under an electron microscope to determine the biological effects of the two metals on the ultrastructure. The most obvious effects of lead were disrupted thylakoidal membranes, accumulated polyphosphate bodies and vacuoles, and lead precipitates on the cell surface. These ultrastructural alterations were partially present in aluminum-treated and lead-aluminum-treated cells. In joint exposure, the most important change was the lysis of the cell membrane. Aluminum and lead seem to act synergistically on the cell membrane leading to cell membrane lysis.