Abstract: A method and system for use in treating a patient with FK 506 to prevent an adverse immune response, either a rejection of a transplanted organ or an attack on the patient's body by its own immune system caused by an autoimmune disease, without unduly suppressing the ability of the patient's immune system to combat infection. The method employs an expert system to provide non-numerical information concerning the course of action to be taken with respect to a patient's FK 506 treatment, at a given time, including, in particular, an instruction on whether the FK 506 dosage being administered to the patient should be changed, and if so how. That instruction is determined by the expert system through analysis of standardized characterizations assigned to standardized patient examination criteria provided to the expert system. The method also employs a means for determining the actual numerical FK 506 dosage the patient should receive at a given time.