A truck-mounted spectrographic laboratory has been designed and built by the U. S. Geological Survey to investigate the feasibility of using and transporting such equipment in the field as an aid in supplying rapid on-the-spot analytical data to geochemical exploration field parties.The laboratory is housed inside a 7- X 12-foot insulated and air-conditioned van-type truck body and carries complete equipment for making qualitative, semiquantitative, and quantitative analyses of geological materials. The spectrograph is a fixed-position 1.5-meter grating instrument of the Wadsworth type which records a range of spectra from 2063 to 4837A in the second order on a 20-inch strip of film. Companion units are a projection type comparator-densitometer, a film processor, and other accessory equipment. Trailer-mounted motor-generators supply 230 volts d-c for the arc source unit and 115 volts a-c for lights and accessories.Since its completion in May 1955, the truck-mounted laboratory has been driven over 4,000 miles. During this travel all the equipment remained in good adjustment; the laboratory was made ready for operation in less than 2 hours after arrival at the site of a field project.Because of the large number of elements that can be determined from a single spectrogram, the truck-mounted laboratory is useful in the early stages of a geochemical exploration project to determine diagnostic suites of elements, and later, to guide the day-to-day sampling.