To a stirred blood sample (3-10 ml) were added 5-10 ml 14.2M-H₂SO₄. A N₂ stream (250 ml/min) was then passed through the mixture and the H₂S produced over 15 min was absorbed in 2-5 ml of 0.5M-NaOH. A portion of the alkaline sulfide solution was diluted to 50 ml with H₂O, after which a known amount of 1% NaN₃ and sufficient 0.1M-HCl to obtain a pH of 6.5-6.8 were added. The solution was stirred and 10 ml 0.02M-I in 1M-KI were added. After 60 s, excess I was back-titrated with 5mM-sodium arsenite using starch as indicator. The amount of I consumed was directly proportional to the sulfide concentration. The calibration graph was linear from 1-500µM-sulfide and the detection limit was 4 µg/l. The RSD (n = 6) were <2%. Recoveries were >98%. The results agreed with those obtained using an ISE.