A rapid field method for the determination of readily-extractable copper in soil and sediment has been developed for use in geochemical prospecting. A 0.2 g sample is treated with 1 ml of cold 6 N hydrochloric acid for 30 seconds; the copper that is extracted is estimated by means of 2,2'-biquino-line in an isoamyl alcohol extract. A sensitivity of 1 ppm is obtained when a 0.2 g sample is used. The percentage extraction is usually great enough to give a positive test on most background materials, a desirable feature.Because a single determination takes only about 2 minutes and requires only simple, easily portable items of equipment, the method is well adapted to use under rough field conditions---at the sample site if desired. Advantages over a modified dithizone procedure for citrate-soluble copper are the much greater relative stability of the biquinoline reagent and copper complex under field conditions, and the fact that biquinoline is specific for copper.Since 1955 the method has been tried on several hundred samples of soil and sediment from various localities in the United States with generally excellent results; the only serious interference encountered was in samples containing considerable amounts of decomposing organic matter.It is suggested that this may be a useful method for those prospectors who are trying to find and outline economically important dispersion patterns of copper in soil or sediment.