Rabbit antibodies against albumin-acetaldehyde adduct were used in an enzyme-linked immunosorbent assay to detect acetaldehyde-hemoglobin condensates from the blood of 12 volunteers following ingestion of 1.3 to 2.9 g of ethanol per kg body weight during 8 hr. Blood samples were drawn before drinking and between 2 to 46 hr after starting the drinking session. While there were no significant increases in blood acetaldehyde levels in these samples, acetaldehyde-hemoglobin adducts were significantly increased in the samples drawn after ethanol had been eliminated from the body. Administration of ethanol (0.1 g/kg) to an Oriental flusher resulted in an increase both in blood acetaldehyde and the hemoglobin-acetaldehyde adduct levels. These results suggest that acetaldehyde-hemoglobin condensates are formed in vivo following acute ethanol ingestion. Such condensates may be of value to mark alcohol consumption.