Iridium concentrations in several types of marine organisms from the northern Gulf of Mexico averaged 20 parts per trillion (range < 4-80 pptr). Based on this value the Pt/Ir ratio for marine organisms is 10 as compared to the 100 that has been reported for both seawater and authigenic Mn nodules. This low ratio may be related to the weaker ability of Ir to form stable chloro-metal complexes in seawater. The Ir inventory in the present-day marine biosphere is five orders of magnitude less than that of Cretaceous/Tertiary (K-T) boundary sediments, making it unlikely that the Cretaceous marine biosphere was a significant source of Ir for K-T boundary sediments. This small, modern inventory also suggests that the recently reported Ir enrichment (1100 ppb) in organic kerogen from K-T boundary sediments probably occurred after deposition as a result of Ir redistribution within the sediments.