At first the author makes a short and critical review of vocabulary tests undertaken in the course of research on the lexical knowledge of children and young people. The author defines the principles of the test's construction and pays attention to those criteria of the test which may have a strong hold on their diagnostical value. Next the author outlines the methods of his own researches on the comprehension of text-books and school literature by the pupils (on the example of the fourth grade of an Elementary School) from the view point of contents of lexical knowledge and describes among others the way of taking advantage of parametricalessential tests. ; With the help of one of these tests, the so-called "Student's test", which is a lexical test and contains a vocabulary assembly of 3 groups (A. vocabulars of a low text-book frequency without any interpretation or comment, B. vocabulars of low frequency but with interpretation or comment, C. vocabulars of high frequency) it was possible to prove the interdependence between the level of text-book and such factors as vocabulary frequency in text-books and school literature, vocabulary interpretation in text-books and the degree of condensation of the text with "difficult" words of special or archaic meaning and so on. ; The author proposes to base the writing of text-books for pupils and the choice of school reading on more rational grounds and put to use informations about the frequency of the applied vocabulary as well as informations about the perceptional possibilities of pupils on the given level of teaching. ; In order to realize those propositions arises the necessity of not only usual frequency dictionaries of contemporary language - which are available - but also of frequency dictionaries of our text-hooks rund pupil`s dictionaries with a subdivision into different ranges of teaching. Such kind of dctionaries are not yet available.