The syntheses of monomeric nucleosides and 3'-O-phosphoramidite building blocks en route to alpha-L-ribo-configured locked nucleic acids (alpha-L-LNA), composed entirely of alpha-L-LNA monomers (alpha-L-ribo configuration) or of a mixture of alpha-L-LNA and DNA monomers (beta-D-ribo configuration), are described and the alpha-L-LNA oligomers are studied. Bicyclic 5-methylcytosin-1-yl and adenin-9-yl nucleoside derivatives have been prepared and the phosphoramidite approach has been used for the automated oligomerization leading to alpha-L-LNA oligomers. Binding studies revealed very efficient recognition of single-stranded DNA and RNA target oligonucleotide strands. Thus, stereoirregular alpha-L-LNA 11-mers containing a mixture of alpha-L-LNA monomers and DNA monomers ("mix-mer alpha-L-LNA") were shown to display DeltaT(m) values of +1 to +3 degreesC per modification toward DNA and +4 to +5 degreesC toward RNA when compared with the corresponding unmodified DNA-DNA and DNA-RNA reference duplexes. The corresponding DeltaT(m) values per modification for the stereoregular fully modified alpha-L-LNA were determined to be +4 degreesC (against DNA) and +5 degreesC (against RNA). 11-Mer alpha-L-LNAs (mix-mer alpha-L-LNA or fully modified alpha-L-LNA) were shown in vitro to be significantly stabilized toward 3'-exonucleolytic degradation, A duplex formed between RNA and either mix-mer alpha-L-LNA or fully modified alpha-L-LNA induced in vitro Escherichia coli RNase H-mediated cleavage, albeit very slow, of the RNA targets at high enzyme concentrations.