Vertebrate chromosomes terminate in variable numbers of T ₂ AG ₃ nucleotide repeats. In order to study telomere repeats at individual chromosomes, we developed novel, quantitative fluorescence in situ hybridization procedures using labeled (C ₃ TA ₂ ) ₃ peptide nucleic acid and digital imaging microscopy. Telomere fluorescence intensity values from metaphase chromosomes of cultured human hematopoietic cells decreased with the replication history of the cells, varied up to six-fold within a metaphase, and were similar between sister chromatid telomeres. Surprisingly, telomere fluorescence intensity values within normal adult bone marrow metaphases did not show a normal distribution, suggesting that a minimum number of repeats at each telomere is required and/or maintained during normal hematopoiesis.