The amino functions of proteins are susceptible to modification by the carbonyl moieties of aldehydes that are found in vivo, such as formaldehyde, glyceraldehyde, acetaldehyde, and glucose. Formaldehyde reacts fairly nonspecifically with proteins. However, under controlled conditions, it may be used as a structural probe to supply the carbonyl component for the reductive alkylation of proteins. Glyceraldehyde reacts less extensively with proteins, as exemplified by its reaction with hemoglobin, and has therapeutic potential as an antisickling agent. Recent work has focused on the modification of hemoglobin by acetaldehyde. This primary metabolite of ethanol metabolism is elevated in alcoholics. Clinical studies on fast-chromatographing hemoglobins (hemoglobin A1a–c) have suggested that a novel adduct may be formed in persons abusing alcohol. Alcoholic patients have an elevated concentration of minor hemoglobins but normal or subnormal amounts of glycosylated hemoglobin (hemoglobin A1c) as determined by radioimmunoassay. Acetaldehyde was found to form adducts with hemoglobin (HbAAA), both in vitro and in vivo, leading to a change in Chromatographic properties. Browning occurred with higher concentrations of acetaldehyde. HbAA formation thus provides a potential marker for alcoholism in addition to providing a biochemical hypothesis regarding the secondary sequelae of alcohol intake.