Anabaena inaequalis was sensitive to nickel ion in the order of decreasing sensitivity of growth, photosynthesis, and acetylene reduction. At a culture density of 9 x 10(4) cells per millilitre, growth after 12 days was completely inhibited by 0.125 ppm (microgram/mL) Ni2+. Nickel caused the increase of both the lag phase of growth and the culture doubling time, and caused the retardation phase to be sooner. Photosynthesis and acetylene reduction were completely inhibited by 10 and 20 ppm Ni2+, respectively, at a cell concentration of 1.3 x 10(6) cells per millilitre. Preincubation for 24 h in the presence of nickel ion significantly increased the sensitivity of photosynthesis and acetylene reduction. Under these conditions acetylene reduction was more sensitive than photosynthesis. Nickel ion reduced culture growth by 35% at a level of 0.05 ppm and inhibited that culture's acetylene-reducing ability by 29% while leaving photosynthesis unaffected. Nickel caused some damage to filament apical cells and induced pigment bleaching in aged cultures. Nickel toxicity was proposed to be due to poisoning of intracellular enzyme systems by nickel ions.