[Truncated abstract] Protein phosphorylation, one of the most important forms of post-translational modification, has been demonstrated to play crucial roles in regulation of cell function. Phosphorylation of protein serine, threonine and tyrosine residues has been the most thoroughly investigated, taking advantage of the acid-stable character of these phosphohydroxyamino acids. Whereas, the cellular occurrence of acid-labile phosphoamino acids, such as phosphohistidine, phosphoarginine and phospholysine was often underestimated due to the acid treatments employed by most of the traditional phosphoamino acid analysis methods. The biological roles of histidine kinases (HKs) in prokaryotes are well understood in contrast to those of HKs in eukaryotes, especially in mammalian cells. However, the evidence has shown that phosphohistidine comprised 6% of phosphoamino acids of the basic nuclear proteins in eukaryotes (Matthews, 1995) and there was more phosphohistidine than phosphoserine in rat liver mitochondria (Bieber and Boyer, 1966). More significantly, phosphohistidine was revealed to be the major phosphoamino acid in phosphorylated histone H4 in regenerating liver in vivo (Chen et al., 1974) and the Walker-256 carcinosarcoma cells in vitro (Smith et al., 1974). Recently, the histone H4 histidine kinase (HHK) activity of human hepatocellular carcinoma (HCC) tumour tissue was measured to be 400 times higher than the normal liver tissue surrounding the tumour. HepG2 cells (HCC cell line) and PIL-2 cells (a p53 knockout mouse tumorigenic liver progenitor cell line) also displayed high HHK activity (Tan et al., 2004). The above observations suggested that HKs and HHKs are playing important roles in both prokaryotes and eukaryotes, including mammals. One major obstacle in the study of HHK study has been the lack of knowledge of the amino acid sequence of an HHK. Attempts at purifying and identifying the HHK from yeast led to the partial purification of a yeast HHK protein(s) at 32kDa (Huang et al., 1991). However, the amino acid sequence of the HHK has not yet been established. ... The success of the separation was demonstrated by the MALDI-TOF-MS and/or ESI-MS spectra of the RP-HPLC fractions. These achievements suggested that it is possible to detect phosphohistidyl histone H4 in vivo using MS under experimental conditions where phosphohistidine is relatively stable. The study in this thesis represents the progression of HHK research in various aspects, including the yeast HHK purification and identification, mammalian HHK partial purification and the methodological developments in detecting histone H4 histidine phosphorylation using MS. Furthermore, new information regarding the physical characteristics of yeast HHKs and its potential role in cellular biology have been documented. It is anticipated that knowledge generated in these studies will contribute to the insight and the understanding of the biological significance of HHK in yeast and mammalian cells.