The onset of autogamy is preceded by a 'critical fission', final, irreversible commitment to autogamy occurring at the G1 phase, about 1 h after this fission. In order to ascertain whether cytoplasm from cells committed to autogamy can induce this process in young cells, cytoplasm from P. tetraurelia cells, being 1,5 to 2 h after the critical fission, was transplanted into cells immature for autogamy. None of the 92 experimental cells entered autogamy, while most of the donor sister cells were autogamous. This result indicates that cytoplasm at this stage is ineffective in inducing autogamy, which may be caused either by the lack of the responsible component, or by too small a volume of transplanted cytoplasm, insufficient to overrule the action of autogamous immaturity factor/s. or/s.