The effect of palladium matrix modification on the determination of thallium has been investigated. The effect of palladium combined either with ammonium nitrate or with magnesium nitrate on the sodium chloride interference was systematically studied and was compared with that of the classical sulfuric acid modifier. A combination of 6-mu-g palladium with 100-mu-g ammonium nitrate allows the direct determination of thallium in ten-fold diluted blood against matrix-free standards. The detection limit (3-sigma) in the undiluted blood was calculated to be 25-mu-g.l-1. For the urine analysis the standard addition technique is recommended because a complete thallium recovery for all different urine samples studied was not always obtained.