AbstractThe four fragments of the terminal inverted repeating units of macronuclear DNA from hypotrichous ciliates d(A4C4), d(C4A4C4), d(A4C4A4C4) and d(C4A4C4A4C4) were synthesized in preparative amounts (300 mg) by block condensation using the phosphotriester procedure. At first, partially protected nucleosides and mononucleotides were condensed to dimeric blocks using 2,4,6‐triisopropylbenzenesulfonyl chloride andN‐methylimidazole. Longer blocks were synthesized from the dimers using 2,4,6‐triisopropylbenzenesulfonyl chloride and tetrazole. After each condensation the reaction mixture was fractionated using a silica gel column and a step gradient. After synthesis, cleavage of the protecting groups and gelchromatographical separation, d(A4C4) and d(A4C4A4C4) were obtained already as pure substances. On the other hand, having cleaved the protecting groups partially or totally, d(C4A4C4) and d(C4A4C4A4C4) were separated first on QAE‐Sephadex by a salt gradient in 7 M urea. The remaining impurities were removed by preparative reversed phase HPLC. Sequence as well as purity of the DNA fragments were proved by partial enzymatic degradation and two‐dimensional chromatography of the hydrolysates (fin