Photosynthetic activity in terms of Oz evolution and the growth of Spirulina platensis was stimulated by adding 5-aminolevulinic acid (ALA, 500 mg/l) to photoautotrophically growing cells. After ALA was added to the medium, intracellular accumulations of phycocyanin and chlorophyll were stimulated simultaneously, followed by enhancement of the photosynthetic activities of photosystems I and II, and lastly, growth was promoted. ALA did not directly activate the photosynthetic electron transport system. However, during a 3-h incubation of intact cells with ALA, photosynthetic activity was enhanced.