Interference in the production of arsine from As(III) and As(V) by a number of transition elements has been eliminated by the use of a 3per thousand solution of L-cystine in 5M hydrochloric acid. With this modification, arsenic can readily be determined at low levels in the presence of up to 10000 mug mlh*-1*s of cobalt(II), iron(II) and(III), and nickel(II). At this level, the signal was reduced by less than 10per thousand0. Interface from silver(I), copper(II), mercury(II), and platinum(IV) is eliminated at levels up to 1000 mug mlh*-1*s. Severe interferences from gold(III) and palladium(II) could not be eliminated above 10 mug mlh*-1*s for gold(III) and 1 mug mlh*-1*s for palladium(II). In the case of mercury(II), a strong memory effect, in addition to the interference effect, was eliminated by the use of L-cystine. Determinations of low levels of arsenic in standard reference iron and steel by the standard addition method showed good recoveries and gave excellent results.