Summary.In this paper a formula obtained by probability analysis is presented. This formula can predict the number of healthy spermatozoa in a sperm population containing more or less numerous different defects recognized in ultrathin sections by transmission electron microscopy or in entire cells stained with different specific antibodies, by fluorescence light microscopy. This formula is able also to orientate the diagnosis of the various sperm pathological affections and, can be used generally in the evaluation of any kind of populations of isolated cells examined in ultrathin sections or in immunocytochemical preparations.