Disclosed herein are antitumor compositions and methods of interfering with the biological activity of PC cell derived growth factor (PCDGF), anti-PCDGF receptor antibodies, fragments thereof, and methods of making anti-PCDGF receptor antibodies. The methods involve inhibiting the proliferation of tumor cells expressing the PCDGF receptor by contacting the surface of the cell with anti-PCDGF receptor antibodies. Anti-PCDGF receptor antibodies are capable of binding to the surface of a cell and interfering with the binding of PCDGF to its receptor. Also provided are compositions comprising anti-PCDGF receptor antibodies and cytotoxic molecules (e.g., toxins, oncotoxins, mitotoxins, immunotoxins, and antisense oligonucleotides).