In this paper we test the predictions of the differential polarization imaging theory developed in the previous two papers. A characterization of the patterns of polymerization of hemoglobin in red blood cells from patients with sickle cell anemia is presented. This system was chosen because it is relatively easy to handle and because previous studies have been done on it. A differential polarization microscope designed and built in our laboratory was used to carry out this study. This microscope uses an image dissector camera, a photoelastic modulator, and a phase-lock amplifier. This design represents a substantial modification with respect to the instrumentation used in the previous results communicated on this system. Therefore, the results presented here also permit us to confirm the validity of our conclusions. On the basis of the differential polarization images obtained, models of the patterns of polymerization of the hemoglobin S inside the sickle cells are proposed and their M12 and regular images are calculated by the theory. Good agreement between those models and the experimental systems is found, as well as with the results previously reported. © 1987, The Biophysical Society. All rights reserved.