G-actin was isolated from human myocardium, purified by means of repeated polymerization-depolymerization and immobilized in polyacrylamide gel by the procedure of "emulsion granulation". The granules acquired magnetic properties after addition of iron protoxide hydrophilic particles into the polymerizating mixture. Immobilized G-actin in granules with magnetic properties was used for production of specific antisera, for isolation of pure antibodies by means of an immunosorption procedure, for estimation of specific antibodies in blood sera of patients with rheumatism, of healthy persons and of experimental animals using immunofluorescent, immunoenzymatic and radioimmune procedures.