A time-dependent set of line scans across an erythrocyte was recorded by a phase contrast laser scanning microscope. A method for edge detection, based on a two-stage fitting procedure of the theoretical intensity distribution in a line scan of phase contrast image to the experimental one, is suggested. Time-resolved fluctuations of the human erythrocyte's edge were obtained, employing this method. The same procedure was carried out for a metallic strip yielding a test for the background noise level. The suggested method was applied in examining the effect of chemical crosslinking and thiol reduction on cell membrane fluctuations of human erythrocytes.