A procedure is described for conjugating the hydrazone derivatives of doxorubicin having a maleimide terminal group to lactosaminated human albumin (L-HAS). The procedure is based on the use of trialkylphosphines to reduce the disulfide bonds of the protein and make its SH groups available for the formation of the thioether bond. In comparison with the conjugation obtained by using thiol reducing agents, such as dithiothreitol, this has the advantage that even when it is performed under very simple conditions, specifically without using an inert atmosphere, in the absence of oxygen, and without preliminary purification of the “reduced” L-HAS, it does not bring about the formation of a precipitate in the reaction means. In comparison with conjugation to L-HAS thiolated by using iminothiolane, the novel procedure has the advantage of greater simplicity and of not introducing exogenous molecules into the L-HAS in order to make the SH groups available.