A new method of estimation of the distance between spins of the spin-label and paramagnetic center is suggested. Method is based on the quantitative analys of saturation curve of spin-label EPR spectra at 77 degrees K. New approaches have been tested using haemoglobin labeled on SH-groups with various iminoxyl radicals. Values of the distances between labels and haem estimated from the saturation curve parameters and by current methods and values of distances estimated from X-ray data are in good agreement. In the case of rapid spin relaxation of he paramagnetic center, the new method allows one to determine a farther distance. Results of the present work make it possible to investigat by the spin-label technique the structure of haem-containing propeins and the structure of other proteins with known spin relaxation time of paramagnetic center.