Noting that four of the six most common readability formulas use the McCall-Crabbs Standard Test Lessons in Reading as the independent criterion of difficulty to establish validity, while the other two often site their correlation with the first four to determine validity, a study examined whether these lessons consisted of passage selections covering a sufficiently wide range of reading difficulty to warrant their use in the development of readability formulas. It was hypothesized that the lessons should increase in difficulty from the beginning of any given booklet to the end, and from Book A to Book D. However, results of analysis showed that the average number of sentences and the average number of syllables per 100 words did not support the assumption that the complexity of the material increased within books or from Book A to Book D. Among the formulas there is considerable variation in grade level designation for the reading selections. The results increase concern about relying on any formula that uses the McCall-Crabbs Standard Test Lessons in Reading as a criterion for development or that bases its validity upon the correlation with other formulas as its justification for application. The results also indicate that readability formulas do not provide school librarians with the information needed to match a given book with a particular student or group of students. (Tables of data are included.) (HTH)