SDS-PAGE analysis of purified RFB4(dsFv)PE38. A total of 2 μg of immunotoxin were loaded in each lane. Reduced: RFB4(dsFv)PE38 was reduced with 10 mmol/L DTT. The variable portion of the heavy chain fused with PE38 migrated with an apparent molecular mass of 48 kD. The light chain migrated faster than the 14.3 kD standard. MW, molecular mass standards. Nonreduced: nonreduced RFB4(dsFv)PE38 migrated with an apparent molecular mass of 58 kD.