It is well known that ammunition containing depleted uranium (DU) was used by NATO during the Balkan conflict. To evaluate the DU origin (natural uranium enrichment of or spent nuclear fuel reprocessing) it is necessary to check the presence of activation products (236U, 239Pu, 240Pu, 241Am and 237Np, etc.) in the ammunition. Uranium isotopes, 239Pu, 240Pu, 241Am concentrations in DU were determined in the past in our Laboratory: the present paper describes a new and accurate method for the separation and determination of 237Np ultratraces in a DU penetrator. After sample dissolution, neptunium was separated by two chromatographic columns consisting of micropourous polyethylene (Icorene) supporting tri-n-octylamine (TNOA); after elettrodeposition, Np was counted by alpha spectrometry. The decontamination factor of neptunium from uranium was higher than 106; the detection limit was 0.5 Bq/kg for 0.10 g of ammunition; the average Np yield was 54.0 ± 3.1%. 237Np concentration in DU resulted 30.1 ± 2.4 Bq/kg. The presence of this anthropogenic radionuclides in the penetrator indicates once more that at least part of the uranium originated from the reprocessing of nuclear fuel, but, because of the very low concentration, its radiotoxicological effect is negligible.