An alternative method for the preparation of human serum albumin-methotrexate derivative (HSA-MTX) using N-hydroxysuccinimide ester of methotrexate (MTX) was compared with that using the water-soluble carbodiimide (WSC) assistance. The prepared derivative was tested to find the advantages and drawbacks of this method. The N-hydroxysuccinimide ester method is more laborious than that using WSC but some reasons speak in favor of this method. The formation of protein-protein conjugates during the reaction was negligible and under specific conditions more MTX molecules could be bound to protein than by the WSC method. However, some disadvantages of this method were found: A laborious preparation, a small degree of denaturation of protein during the synthesis and, moreover, the binding of MTX to protein did not always take place through NH2-groups of protein but through some other groups (--OH, --SH) as well. These couplings were not so stable as an amidic (or peptidic) bond. In water environment they were hydrolyzed and MTX was slowly released. If there is no difference in a small fraction of protein-protein conjugates, the WSC-assisted method possesses evident advantages over the active MTX ester one, mainly because of the simple preparation of the stable derivative.