The role of impaired toxin uptake in conferring cellular resistance t o the plant toxin RCAII (ricin) has been examined using a murine BW5 147 lymphoma line and a toxin-resistant variant (BW5 147RicR.3) selected by repeated exposure t o RCAII. The toxin-resistant variant is 250 times more resistant t o RCAII in long-term growth experiments and 1,000 times more resistant in short-term protein synthesis assays. Experiments with ferritin-conjugated I-labeled RCAII (ferritin-IL5 I-RCAII) indicated that toxin binding t o sensitive and resistant cells is similar at low toxin concentrations where maximum differential cytotoxicity occurs but that major differences exist with respect t o toxin uptake. In sensitive cells toxin is internalized via endocytosis, and as seen previously in other systems subsequent rupture of some of the toxin-containing endocytotic vesicles releases toxin into the cytoplasm, where it inhibits protein synthesis. The process of toxin transfer t o the cytoplasm is presumed t o account for the one-hour lag before toxin-induced inhibition of protein synthesis can be detected. Endocytotic uptake of toxin is impaired in resistant BW5 147RicR.3 cells, and they are unaffected by toxin concentrations that inhibit protein synthesis and kill sensitive parental cells. Killing of resistant cells at low toxin concentrations was accomplished by encapsulating RCAII into lipid vesicles capable of fusing with the plasma membrane. Direct introduction of toxin into resistant cells using lipid vesicles as carriers produced rapid inhibition (< 15 min) of protein synthesis and eliminated the lag in toxin action seen in sensitive cells exposed to free toxin. These findings are discussed in relation t o the mechanism of toxin action and proposals that toxin activity requires structural modification of the toxin molecule at the cell surface before transport into the cell.