The purpose of this article is the interpretation of the clinical value of blood cells' (erythrocytes, leukocytes, thrombocytes) morphological characteristics changes taking into the consideration results of our own long-term (1982-2012) investigations as well as literature data. Patients with hematological and nonhematological diseases as well as large groups of Georgia population (about 30.000 persons, in age 2-81) were observed. By Gimsa-Romanovsky method stained blood smears were studied in light microscope. Changes of blood cells' morphological characteristics, observed by us and known in the literature, as well as ones revealed at first in this work, are presented according to the three branches of hemopoiesis. Possible changes of blood cells' morphological characteristics are described in details. Concrete changes in concrete diseases are presented in eight tables. Discussing the results the fact of the revealing of blood cells' morphological changes in some diseases, different by etiology, pathogenesis, clinical symptomatology and treatment, is accentuated. Some examples of the differentiation of different diseases by means of that nuances, which are revealed in microscope and which are not noticeable for the modern techniques, are presented. The significance of some specific changes of blood cells' morphological characteristics for the doubtless diagnostic of some concrete anomalies is shown. The list of changes which are revealed in this work but are not founded in the literature is presented. The conclusion is drawn that the nuances of blood cells` morphological changes revealed in light microscope allow to narrow the spectrum of additional investigations for the establishment of diagnosis.