The elementary processes of protein crystal growth were investigated by means of laser Michelson interferometry on the example of the (101) face of tetragonal hen egg-white (HEW) lysozyme. The method allows real-time in situ observations of the morphology of the growing protein crystal surface, as well as simultaneous precise measurement of growth rate and step velocity on identified growth-layer sources. At the critical supersaturation of 1.6 the growth mechanism was shown to transform from dislocation-layer generation to surface nucleation. Measurements on different growth hillocks, with material of a different source and at a different temperature, all indicated that for supersaturations lower than approximately 1 growth is hindered by the competitive adsorption of (most probably) other protein species contained in HEW, although the material is pure by most analytical methods. At supersaturations sigma < 0.4 other impurities sometimes led to cessation of growth. However, at sigma in the range 0.9 < sigma < 1.6 growth processes are determined by the kinetics of pure lysozyme. This enabled us to measure the step kinetic coefficient beta for crystallization of a protein substance for the first time: beta = 2.8 micro m s(-1). This also means that by working in this supersaturation range we can eliminate the impurity effects. Other means to reduce influence of impurities is to use, if possible, a higher crystallization temperature. It is shown that slow crystallization of proteins is due primarily to impedance of the elementary act of entering the growth site and not to the low concentration of the solution. The value of beta does not depend on temperature, indicating the decisive role of entropy, not energy barriers, in the crystallization of biological macromolecules.