Abstract: Two sensitive and rapid RIA method for quantitative measurement of free triiodothyronine (FT_3) and free thyroxine (FT_4) in serum are described. These two step RIA procedures utilize immunoextraction of free hormone and separation of free phase by specific, high affinity polyclonal antiserum coupled to cellulose coated magnetic iron oxide particles. The measurement range is 0—100 pmol/l for FT_4 and 0—50 pmol/l for FT_3 respectively. Sensitivity of FT_3 and FT_4 is no more than 0.5 pmol/l. The normal range is 2.5—9.5 pmol/l for FT_3 and 9.5—25.5 pmol/l for FT_4.