Limitations of traditional techniques based on selective enrichment and pure-culture isolation make it difficult to precisely characterize the natural microbial ecosystems. Molecular techniques are now being developed and used to address these limitations. Group-specific 16S rRNAs-targeted oligonucleotide probes of different phylogenetic levels are increasingly used to identify and quantify the microbial members in complex environmental samples. Two domain-specific probes were used in this study to elementarily characterize the defined mixtures of RNAs extracted from pure culture (synthetic samples) and a RNA sample obtained from landfill leachate (environmental sample). The results demonstrated that 16S rRNA quantitative hybridization provided an excellent estimation of domain level community composition of these samples, and thus, had a huge potential of usefulness in microbial ecology studies.