Abstract: Fluorescent dye is added to a sample including lymphatic cells stimulated by antic bodies to measure fluorescence polarization. The fluorescence polarization varies with time. The measurement is stopped after a predetermined measurement time, and the lymphatic cells are separated from the sample by filtering. In order to determine the fluorescence polarization in a filtering process, approximate equations of the reaction are determined based on the fluorescence polarization measured during the reaction process. Based on the approximate equations, the fluorescence polarization in the filtering process is calculated. The reaction process is approximated by both an approximate linear line and an approximate curve. A reaction data at a predetermined future time is calculated based on the two approximate functions so that a reliability of the resulting data is improved.