In order to estimate the transfer of amoxicillin in the body fluids, a direct separatory determination method of amoxicillin and its penicilloic acid was investigated. The penicilloic acid of amoxicillin formed an intensive and reproducible fluorescent product in solutions containing Hg Cl2, but the fluorescent product was not formed from amoxicillin directly. The measurement can be made of amoxicillin and its penicilloic acid separately in urine and blood following therapeutic doses of the drug. The blood levels and urinary excretion of amoxicillin were compared with those of ampicillin.