A method is described for preparing argatroban monohydrate obtained from (2R,4R)-1-[NG-nitro-N2-(3-methyl-8-quinolinesulphonyl)-L-arginyl]-4-methyl-2-piperidine carboxylic acid by suitably treating crude argatroban. The method either comprises preparation of argatroban monohydrate in a continuous step or an intermediate step of isolating a purified argatroban. Also obtainable from argatroban monohydrate is anhydrous argatroban, shown to have new physico-chemical characteristics.The described argatroban synthesis and purification process hence enables three different forms of argatroban, not previously described, to be obtained, each with distinctive physico-chemical characteristics and in particular enables argatroban monohydrate to be obtained with high yield and with high purity, being therefore a product suitable for use as active principle in proprietary medicines.