The administration of Reaferon, the analog of human alpha 2-interferon obtained by means of gene engineering technigues, to 25 volunteers revealed the ability of small doses of this preparation to produce a stimulating effect on the capacity of lymphocytes and neutrophils for rosette formation. When introduced in large doses, the preparation sharply suppressed this capacity. A significant decrease in the T- and B-lymphocyte count, as well as in the neutrophil count, was noted. These changes persisted for 7 days after the course of Reaferon injections had been over. The intravenous injection of the preparation produced more pronounced effect than its intramuscular injection in the same doses. No essential changes in the count of the precursor cells of both T- and B-lymphocytes were registered.