Opium alkaloids (thebaine, codeine and morphine) have been conjugated with bovine serum albumin (BSA) to give individual antigen conjugates which are analyzed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. It became clear that 9 molecules of thebaine were contained in a thebaine-BSA conjugate. Codeine and morphine contents in individual conjugates were determined to be 12 and 6 molecules, respectively.