Melphalan, a drug used for the treatment of breast, ovaries and a certain type of cancer in the bone marrow, was conjugated to linear methoxy poly (ethylene glycol) (M-PEG) of 2000 and 5000, Da. An ester linkage between polymer and drug was used in the coupling to yield a polymeric prodrug. Purified esters were characterized by Maldi-Tof and IR spectroscopy methods. The modification allowed overcoming the known melphalan aqueous solubility problem (0.1 µg/ml) leading us to obtain a polymer-drug bioconjugate more suitable for oral and parental administration. It was found that molecular weight of M-PEG is critical for the conjugates stability, aqueous solubility (80 times and 123 times higher aqueous solubility for M-PEG 2000 and M-PEG 5000, respectively), and hemolytic activity. The melphalan caused 100% hemolysis above the concentration 3.5 µg/ml in 1 h. whereas conjugate of M-PEG 2000 and M-PEG 5000 shows 81.3 ± 0.5% and 48.8 ± 1.5% hemolysis, respectively at 32 µg/ml after1 h. Further In vitro anticancer activity of melphalan and its conjugates was performed with breast cancer MCF-7 cell lines. It shows that LD50 concentration was higher 1.14 and 2 µm for M-PEG 2000 and M-PEG 5000, respectively in comparison to pure melphalan (0.74 µm). Above studies revealed improved pharmacokinetics properties upon conjugation.