ATOMIC reactors at Hanford Laboratories, Washington, introduce easily measurable quantities of chromium-51 into the Columbia River. Although in hexavalent form when produced, some of the radioactive chromium is probably reduced to the trivalent state while passing downstream. Chemical species likely to exist are CrO4=, HCrO4-, Cr+3 (aq.) and Cr2O3.nH2O. Observations of Osterberg and Cutshall (unpublished) confirm the presence of several species in the river. They found that 51Cr could be removed from river water using membrane filters, chelating resins and both cation and anion exchange resins. Membrane filters (0.65 μpore size) retained about 20 per cent of the 51Cr from river water collected near Bonneville Dam. Seymour2 noted earlier that filters with a 0.45µ pore size removed about 25 per cent of 51Cr from river water taken near the mouth of the Columbia River. Measurements made during a recent cruise in the lower 210 km of the- river showed that the percentage of 51Cr attached to particles gradually increased downstream3.