[en] Physical and radiochemical properties of labelled microspheres of human serum albumin have been studied as a function of preparation. It is shown that the disperse composition of microspheres depends on the mixing velocity of the reaction mixture. The mean radius of microspheres decreases with increasing mixing velocity. The functional dependence of the activity of microspheres on their size has been determined. This dependence is of square character if microspheres are labelled after their production, and it is of cubic character if they are labelled during production. The number of microspheres per mass unit can be calculated if the parameters of particle size distribution and the kind of dependence of the activity of microspheres on their size are known. It was found that high radiochemical purity of the preparations is preserved also in long-term storage. (author)