The mechanisms of novocain damaging action on blood cells in persons with increased sensitivity to this drug were studied in the leukocyte alteration test. The leading role of histamine liberated from basophils was established. The increased sensitivity to novocain was shown not to be passively transferred to the cells of healthy donors with patient serum with intolerance to the drug; moreover, the joint action of both cell-mediated and thermolabile humoral factors was found to be necessary for the realization of leukocyte alteration under the action of novocain. The comparison of the information content of a number of methods--skin testing, dosed provocation, leukocyte alteration test and chemiluminescence--for revealing increased sensitivity to novocain in 30 persons with adverse reactions to this preparation registered in their medical history. The two in vitro tests were shown to be comparable in their diagnostic significance with the method of dosed provocation.