The present invention concerns methods and compositions for forming immunotoxin complexes having a high efficacy and low systemic toxicity. In preferred embodiments, the toxin moiety is a ranpirnase (Rap), such as Rap(Q). In more preferred embodiments, the immunotoxin is made using dock-and-lock (DNL) technology. The immunotoxin exhibits improved pharmacokinetics, with a longer serum half-life and significantly greater efficacy compared to toxin alone, antibody alone, unconjugated toxin plus antibody or even other types of toxin-antibody constructs. In a most preferred embodiment the construct comprises an anti-Trop-2 antibody conjugated to Rap, although other combinations of antibodies, antibody fragments and toxins may be used to form the subject immunotoxins. The immunotoxins are of use to treat a variety of diseases, such as cancer, autoimmune disease or immune dysfunction.