We have studied the mechanism controlling the behavior of accessory sperm nuclei in physiologically polyspermic eggs of the newt, Cynops pyrrhogaster. Our approach was to identify cytoplasmic components which would prevent the usual degeneration of accessory sperm nuclei. Injection of cytoplasm from unfertilized eggs, but not fertilized ones, induced multipolar cleavage in polyspermically fertilized eggs as well as centrosome separation and formation of extra bipolar spindles in accessory sperm nuclei. Cytosols extracted from unfertilized Cynops or Xenopus eggs also were active in inducing multipolar cleavage, as were germinal vesicle materials from oocytes of the frogs Xenopus or Rana or of Cynops. In all of these cases, the nuclear cycle as well as the onset of first cleavage was delayed relative to those in control eggs. In contrast, injection of an extract with maturation-promoting factor (MPF) activity, prepared from unfertilized Xenopus eggs, induced precocious and multipolar cleavage when injected into fertilized Cynops eggs. Injection of the MPF-containing extract caused acceleration of the nuclear cycle as well as formation of extra bipolar spindles by the accessory sperm nuclei. These results suggest that a local deficiency of MPF may lead to the degeneration of accessory sperm nuclei in physiologically polyspermic eggs.