The cytotoxic activity of cyclosporin A (CsA) and the three non-immuno-suppressive CsA analogues B3-243, WO-039 and B3-665 were studied in tumor cell lines representing both classical and atypical forms of multidrug resistance (MDR): T-ALL GM3639 L100 cells selected for vincristine (vcr) resistance and displaying characteristics of classical MDR, including P-glycoprotein (pgp) expression and increased drug efflux which can be inhibited by pgp blockers (e.g. verapamil), and U-1285/ADR, a small cell lung cancer (SCLC) cell line selected for doxorubicin resistance which lacks pgp, is insensitive to pgp-blockers and shows cross resistance to cis-platinum. At 1 micrograms/ml CsA was the most active agent in reversing Vcr resistance in L100 cells followed by B3-243 and WO-039, with no effect of B3-665. Parental LO cells were only marginally sensitized to Vcr by these agents. No reversing effect of any cyclosporin was observed in the U-1285/ADR or its parental cell line. Compared to LO cells, L100 cells showed a marked hypersensitivity to CsA > B3-243 > WO-039 with B3-665 being inactive. No collateral sensitivity was observed for cyclosporins in U-1285/ADR cells. Although of different magnitude, the pattern of cytotoxic activity for the different cyclosporins alone closely parallelled that of L100 cells for U-1285, U1285/ADR and LO cells. The results indicate that not only the collateral sensitivity in classical MDR but also the cytotoxic actions of cyclosporins per se on tumor cells alone are independent of immunosuppressive activity. The results also suggest a structure-activity relationship for cyclosporin-induced cytotoxicity similar to, but independent of, MDR reversing activity.