The preparative isolation of DNA fragments with defined sequences from hydrolysates of naturally occuring DNA is proposed as an alternative to their complicated synthesis and has been partially realised. Starting with herring sperm DNA, which was depurinated in 100 g batches, one obtains 25 – 40 g of a pyrimidine oligonucleotide mixture, which is separated into high and low molecular weight fragments via column chromatogxaphy on DEAE-cellulose. The pyrimidine nucleotide mixture thus obtained is then selectively separated on a preparative scale into fragments with one to six monomer units by a separation procedure on QAE-Sephadex.