Paramecium tetraurelia cells of ages 4, 15, and 27 days were labeled with [14C]-thymidine. In addition, cells were grown clonally for 27 days (108 generations) and labeled with [14C]-thymidine in the presence of 0.5 or 7.5 micrograms/ml of mitomycin-C (MMC) or no MMC. These cells were gently deposited on a filter membrane, which impedes the passage of DNA strands. The cells were then lysed with detergents and the cellular components washed through the filters, leaving double-stranded DNA intact on the surface. Proteinase K was used to remove histone or DNA-bound proteins. The DNA was then eluted under alkaline conditions, which denatures double-stranded DNA and converts apurinic/apyrimidinic sites into single-strand breaks. The results obtained with the cells of ages 4, 15, and 27 days (16, 60, and 108 generations, respectively) indicate that as Paramecium tetraurelia ages during asexual reproduction, apurinic/apyrimidinic lesions, strand breaks or single-strand gaps accumulate. This accumulation may be the basic mechanism of aging in such cells. In the MMC-treated cells of 27 days (108 generations), the MMC reduced elution of DNA fragments more at the higher than at the lower pH's used; random MMC cross-links should occur more often in longer strands than in shorter strands. The reductions in elution preferentially at higher pH, at which longer single strands would be eluted, confirmed the pH-versus-length relationship for Paramecium DNA eluted under our conditions.