Ricin A chain acts to inhibit protein synthesis only if it penetrates the plasma membrane, and this requires the participation of the B chain. The two chains are held together in ricin by a single disulfide bond. In this paper, it is shown that the addition of A chain to mouse leukemia cells (AKR SL3) after the cells were first reacted with purified ricin B chain also results in efficient inhibition of protein synthesis. The data indicated that B chain bound to the cell surface was capable of causing the transport of A chain. The quantities of ricin and of B chain (in the presence of a saturating amount of A chain) required to inhibit protein synthesis by 50% are nearly identical, indicating that there is little difference in the toxicity to cultured cells between ricin and the A chain-B chain heterodimer formed from purified subunits. However, if the addition of A chain to B chain-treated cells was delayed sufficiently long (90 to 120 min), B chain was no longer able to cause A chain transport and the consequent inhibition of protein synthesis. Direct binding studies indicated that only a fraction of the bound 125I-B chain was internalized during this time. No proteolytic degradation of 125I-B chain could be detected after 3 h incubation with the cells.