Chemically reactive compounds in tissues can be monitored through their products of reaction with biomacromolecules. For the purpose of in vivo dose monitoring, hemoglobin (Hb) has been preferred to DNA because of its well-defined life span and more facile chemical identification of adducts. Through the N-alkyl Edman method, adducts to the N-terminals (valines) of the globin chains are measured mass spectrometrically with high sensitivity. In studies of low molecular weight adducts from occupational exposures or tobacco smoke, background levels were found in nonexposed control persons. In some cases the origin of these adducts could be determined. For instance, the 2-hydroxyethyl adduct has been shown to originate from ethylene oxide, a metabolite of endogenously produced ethene. The measured level, about 20 pmole/g globin, agrees well with the ethylene oxide dose calculated from expired ethene. Animal studies indicate contributions from the intestinal flora and dietary factors. An average background level of about 200 pmole/g globin of methylvaline has been observed in unexposed humans. From reaction-kinetic studies of S-adenosylmethionine (SAM), it has been shown that the background mainly originates from SAM. In twin studies, a genetic influence on the level has been shown. Furthermore, a contribution from tobacco smoking to the level was demonstrated in these studies. Certain aldehydes, e.g., malonaldehyde, have been shown to be related to dietary factors and lipid peroxidation. These studies show the usefulness of the method in a search for reactive compounds in the body, with the ultimate goal of assessing the total genotoxic load.