The interpretation of clinical specular microscopical images of abnormal corneal endothelium remains a problem in many instances when the relief mode is negative. This is primarily because different cellular changes occurring simultaneously present similar appearances. In this paper is compiled a number of specular images of small, dark (non-reflecting) cell changes in the endothelium which resemble one another and clinical examples, that have been induced in rabbit eyes by various means. Specific cells showing these changes have been relocated by scanning electron microscopy and orientated to match specular photomicrographs taken before fixation. This has allowed precise comparison of their specular and scanning electron microscopical appearances and the interpretation of the former by the latter. It is found that reversible changes are expressed as an excess of microvilli on the cells and minor inclination of the posterior cell membrane from the perpendicular with the axis of the microscope, and irreversible changes are due to cell fragmentation or total loss with exposure of Descemet's membrane to the aqueous humour. Re-examination of specular photomicrographs with this knowledge in mind indicates that very subtle differences in the specular images often betray the true nature of the cell change. An exception to the rule is included.