A unique method of potentiating the effect of anti-cancer immunotoxins has been developed, thus offering to significantly improve the treatment of a number of cancers as well as autoimmune diseases. Prolonged treatment of human cancers with classical methods such as radiation and chemotherapy, or a combination of both, may cause greater damage than the underlying disease because healthy tissue is often damaged along with diseased tissue. More recently, immunotherapy has emerged as a new and promising therapy for treating cancer because it employs monoclonal antibodies specific for tumor cells coupled to protein toxins. Thus, cancer cells are selectively targeted for destruction. These immunotoxins are being examined in numerous clinical trials for the treatment of cancer and autoimmune diseases. However, often the protein toxin coupled to the monoclonal antibody does not pass as readily into the cytosol of the target cell as does the native protein toxin. This invention improves the effectiveness of such immunotoxins by employing retinoic acid, which disrupts the Golgi apparatus of the target cell and increases the cytosolic routing of specific protein toxins. Also included in this invention is an in vitro method for assessing the ability of a retinoid to potentiate the activity of immunotoxins.