Blood cell production involves the commitment and differentiation of hematopoietic stem cells to committed progenitor cells which undergo a programmed development to form mature cells such as neutrophils, macrophages and lymphocytes. This complex process can be disrupted in diseases such as the leukemias and myeloproliferative disorders by oncogenes such as protein tyrosine kinases. The analysis of expression patterns for specific genes suggests that the regulation of protein expression can be achieved in a posttranslational fashion. Post-translational protein modification, such as phosphorylation and acetylation govern events in blood cell production and yet cannot be measured using conventional molecular biology approaches. For this reason a suite of techniques in mass spectrometry needs to be applied to define regulation and disregulation in normal and abnormal hematopoiesis. These approaches include discovery proteomics with relative quantification of thousands of proteins. Alternatively targeted examination of a single protein to identify its interaction partners or post-translational modifications using mass spectrometry reveals much mechanistic detail. The use of mass spectrometry and proteomics approaches in stem cell and leukemia studies has thus far revealed a good deal of information on hematopoiesis. Further application of the proteomics approach is a necessity to gain true insight into regulatory processes governing the production of billions of blood cells a day, and ways in which that process can be manipulated to therapeutic advantage.