Carl Woese uses ribosomal RNA analysis to recognize a third form of life, the Archaea, whose genetic makeup is distinct from but related to both Bacteria and Eucarya.
Woese, C. R. and G. E. Fox. 1977. Phylogenetic structure of the prokaryotic domain: The primary kingdoms. Proc. Natl. Acad. Sci. USA 74: 5088-5090. In Microbiology: A Centenary Perspective, edited by Wolfgang K. Joklik, ASM Press. 1999, p.440 [pdf]
G.E. Fox, K.R. Pechman, and C.R. Woese. 1977. Comparative cataloging of 16S ribosomal ribonucleic acid: molecular approach to procaryotic systematics. Int. J. Syst. Bacteriol. 27:44-57. In Microbiology: A Centenary Perspective, edited by Wolfgang K. Joklik, ASM Press. 1999, p.264 [pdf]