In the perspective of increasing the clinical potential of ricin A chain immunotoxins (RTA-ITs), perhexiline (Pex) and four structural analogues (Pex 2, Pex 3, Pex 7, and Pex 11) were evaluated for their ability to enhance RTA-IT activity in vitro. Only perhexiline significantly enhanced the cytotoxic activity of anti-CD5 RTA-ITs, T101 and T101-F(ab')2, on CEM III cell line (30- to 2000-fold), and of anti-HLA-DR RTA-IT, HNC-241, on both RAJI cell line (greater than 100-fold) and two immortalized cell lines originating from patients suffering from B-cell chronic lymphocytic leukemia, EHEB and FS2 D5 (10-fold). On 16 consecutive fresh B-cell chronic lymphocytic leukemia cell samples, significant T101-F(ab')2 RTA-IT and HNC-241 RTA-IT enhancement was observed with perhexiline which was comparable to that of NH4Cl and monensin. Perhexiline almost completely blocked RTA-IT intracellular degradation and profoundly modified its routing. These observations were linked to perhexiline-induced lipidosis via inhibition of sphingomyelinase activity. In conclusion, since the concentrations used are relevant with the pharmacokinetics of this agent, perhexiline appears to be a promising agent for in vivo enhancement of ricin A chain immunotoxins.