The aim of this study was determination of the cata ly tic activities of adenosine deaminase (ADA), ADA1 and ADA2 isoenzymes in the serum of patients suffering from rheumatoid arthritis (RA) who were and were not treated with methotrexate (MTX), and identification of the possibilities of using these biochemical parameters in diagnosing and monitoring the treatment effects in RA. Catalytic activities of total ADA (tADA) and ADA2 in serum were determined by a spectrophotometric method. A statistically significant correlation was found between the total ADA and ADA1 values, as well as between tADA and ADA2 in the serum of all patients suffering from RA. Determination of ADA1 and ADA2 isoenzyme catalytic activities in the serum of patients who might be suffering from RA improves the diagnostic value of total ADA catalytic activity determination. ADA2 catalytic activity in serum can be a useful biochemical marker in diagnosing and monitoring RA. Decrease in ADA1 isoenzyme catalytic activities in the serum of patients suffering from RA who were treated with MTX can help in the observation of MTX therapy effects.Kratak sadr`aj: Cilj rada bilo je odre|ivanje kataliti~kih aktiv nosti izoenzima adenozin deaminaze (ADA), ADA1 i ADA2 u serumima obolelih od reumatoidnog artritisa (RA) le~enih uz pomo} ili bez terapije metotreksatom (MTX), kao i utvr |i vanje mogu}nosti uvo|enja ovih biohemijskih parametara u dijagnostiku i pra}enje terapijskih efekata u RA. Kataliti~ke aktivnosti ukupne ADA (tADA) i ADA2 u serumima odre|i vane su spektrofotometrijskom metodom. Utvr|e na je stati sti ~ki zna~ajna korelacija izme|u kataliti~kih aktivnosti tADA i ADA1, kao i tADA i ADA2 u serumima svih ispitanika sa RA. Odre|ivanje kataliti~kih aktivnosti izoenzima ADA1 i ADA2 u serumima pacijenata sa sumnjom na RA pobolj{ava dijagnosti~ku vrednost odre|ivanja kataliti~ke aktivnosti tADA. Kata li ti ~ka aktivnost ADA2 u serumu mo`e da bude koristan bio he mijski marker u dijagnostici i pra}enju RA. Sma njenje ka ta li ti~kih aktivnosti izoenzima ADA1 u serumima pacijenata sa RA koji su dobijali MTX mo`e da pomogne u pra}enju terapijskih efekata MTX.Klju~ne re~i: izoenzimi adenozin deaminaze, metotreksat, reumatoidni artritis