The effect of activated charcoal (AC) on rifampicin excretion was investigated in six healthy volunteers. On three occasions, at weekly intervals, each subject received a 600 mg rifampicin with 350 ml of water; b+c one and two weeks later, 600 mg of rifampicin plus 7.5 and 15 g AC, respectively, in 350 ml of water as a charcoal slurry. Urinary levels of rifampicin were measured form 1-36 hr after ingestion. Treatment with AC led to 1.2 per cent urinary recovery of rifampicin; decreased excretion rate; and a much lower plateau indicative of reduced absorption.