Testing for erythrocyte sickling is a common procedure in many laboratories, and it is generally considered simple to perform and interpret. Nevertheless, we frequently receive reports which are found to be falsely positive or falsely negative, and such incorrect data have even been recorded in connection with investigations on the abnormal hemoglobins.1Recent discoveries of the existence of several dozen hereditary hemoglobin variants and the development of methods for their identification have made possible the recognition of many previously unknown hemoglobinopathies. Although such recognition depends largely on electrophoretic and genetic data, sickling tests often provide a valuable rapid diagnostic aid. The purpose of this paper is to describe conditions under which the reliability of these tests is frequently impaired. Precautions for Accuracy.— Obviously, the blood sample for testing must be the patient's own, unmixed with that of a donor. Yet, we receive a surprising number of samples which were