Abstract: This invention provides a method of determining the behavior of drugs in living bodies, particularly those which contain a variety of active ingredients, like biological, natural drug and crude-drug preparations. It comprises administering a drug being tested to an animal, taking a blood sample from that animal, deproteinizing the plasma separated from the blood, intravenously administering the deproteinized plasma thus prepared to a different test animal, and examining the effect of drug exhibited in that test animal. The amount of active ingredients absorbed into the blood can be estimated by this indirect method even with special drugs including a variety of unknown and trace components.