The contribution of the macronucleus of Paramecium tetraurelia to the long term proliferation potential of a cell line was tested using a nuclear transplantation protocol. Macronuclei from young or old wild-type cells were injected into genetically marked host cells of a standard clonal age. The subsequent proliferation to clonal death of successfully and stably transformed hybrids was compared to the proliferation of injected but untransformed lines (injection controls). Young macronuclear donor material significantly prolonged the proliferation of the hybrid cell lines over that of the injection controls, but old donor material caused only a slight increase in post-injection proliferation of the hybrids. Total cell proliferation was also considered. Comparison of the total life spans of injected, non-transformed lines with uninjected host controls demonstrates that the injection itself has no significant effect upon proliferation potential. The mean life spans of uninjected donor controls and the mean total life spans of the transplanted macronuclear material (donor age at injection plus subsequent hybrid proliferation) are similar, regardless of the age of the donor at the time of transplantation. These results suggest that there is an age-related decrease in the ability of a macronucleus to support subsequent cell growth and division. The results also show that the proliferation potential of the donor macronucleus does not appear to be changed to any great degree by transplantation into a host cell of different clonal age. The macronucleus thus "remembers its age" after transplantation. Coupled with an absence of any detectable cytoplasmic effects upon aging during vegetative growth, these results argue in favor of a macronuclear determination of the proliferation potential of a cell line. The identification of a macronuclear basis for clonal aging in P. tetraurelia should permit a better directed approach for further research in this area.