T4 RNA ligase has been used to construct a series of defined oligoribonucleotides. Hexamer or pentamer blocks were synthesized first by multiple additions of mononucleotide diphosphates to trimers with T4 RNA ligase and removal of the terminal phosphate with alkaline phosphatase; inhibitors of the ligase were removed by passing the sample over a 1-ml reverse-phase octadecasilyl column. The two nucleotide blocks were then ligated to give undecamers. Yields for the individual ligations ranged from 85 to 100% for acceptors lacking uridines and at least 70% for those containing uridines. The overall yield of the undecamer relative to the starting trimers was about 10%. Each round of ligation averaged about 8 h; the time required to synthesize each undecamer was 1 to 2 weeks. Optimization of the steps to achieve this is described in detail.