The laboratory diagnostic process of drug intolerance is a clinical challenge. The allergic leukocytic alteration test (ALAT) was used to detect drug intolerance and to estimate its informative value. Three hundred and eighty-seven patients with various manifestations of drug allergy were examined (1227 studies). The test was carried out in vitro using heparinized blood, calculated with reference with 1 ml per drug and incubated at 37 degrees C for 1.5 hours. Smears were prepared, stained by the Romanovsky-Giemsa method, and examined by light microscopy. A hundred cells (neutrophils) were scanned to evaluate allergic alteration. When the affected neutrophils were more than 10%, the test was considered as positive. The findings suggest that ALAT reveals intolerance of different groups of drugs with a high degree of significance and may be proposed for the diagnosis of drug allergy as it is of great informative value, safe to a patient, and allows a large number of drugs to be tested.