Centriolar complexes isolated from sperm of the starfish, Asterina pectinifera, could initiate irregular cleavage by injecting them into the fertilized eggs of the sand dollar, Scaphechinus mirabilis. Unfertilized sand dollar eggs could be activated when Ca2+ concentration in the egg cytoplasm increased by means of microinjecting calcium buffers in which the concentration of free Ca2+ was controlled by mixing calcium salt and calcium chelator at various proportions. When the centriolar complexes were injected into Ca2+-activated eggs, aster formation was induced in 25 out of the 55 eggs injected with calcium buffers at an intracellular Ca2+ level above 2.9 μM. The number of asters detected in one egg was quite variable, ranging from 2 to 20. The pronucleus in injected eggs was seen to migrate towards the induced asters. Eight of those 25 eggs cleaved.