The effect of orally administered activated charcoal on the absorption of acetylcysteine in three men was studied. Each of the three volunteers was given 140 mg/kg acetylcysteine as a single oral dose. Blood samples were taken at 0, 0.5, 1, 1.5, 2, 4, 6, and 8 hours after acetylcysteine ingestion. Following a one-week washout period, each subject was given orally 50 g of Activated Charcoal, USP, 15 minutes before a 140-mg/kg acetylcysteine oral dose. Blood samples were taken. Acetylcysteine content in the plasma was determined with a high-pressure ion-exchange chromatography system using a gold-mercury electrochemical cell. No statistically significant differences were found between total areas under the curves, peak concentrations, or time of peak concentrations of acetylcysteine given with or without charcoal. Although acetylcysteine absorption was delayed in two patients, the differences were not significant. These data suggest that acetylcysteine absorption is not impaired by activated charcoal administration. This conflicts with previously published in vitro data; therefore, it is recommended that activated charcoal should not be administered concomitantly with acetylcysteine as a usual procedure until more data are available.