Ricin, a type II ribosome-inactivating protein, has been used as a biochemical weapon due to its ability to inhibit protein synthesis and induce cytotoxicity. The unfolded protein response (UPR) is a survival response that helps cells to recover from stress that occurs following the accumulation of misfolded proteins in the ER. Failure to recover from ER stress can lead to apoptosis. In yeast, ricin-A-chain (RTA), the enzymatic component of ricin, inhibits the unfolded protein response (UPR). However, the ability of RTA to affect UPR has not been investigated in mammalian cells. The goals of this project were to determine if RTA could inhibit the UPR in mammalian cells and if altering the UPR affected RTA cytotoxicity. The UPR consists of three signaling cascades, IRE1, PERK, and ATF6. In HeLa cells, a human cervical carcinoma cell line, we found that RTA could inhibit tunicamycin (Tm)-induced IRE1 phosphoryation and XBP-1 mRNA splicing at 4 h. A similar effect was observed in the non-transformed mammary epithelial cell line MAC-T. RTA also inhibited the ability of dithiothreitol (DTT) to activate the PERK pathway in HeLa cells, as shown by inhibition of DTT-induced eIF2α phosphorylation at 4 h. In both cell types, RTA decreased expression of the downstream chaperone BiP in response to Tm. To determine if inhibition of the IRE1 and PERK pathways by RTA affected its cytotoxicity, cells were treated with RTA in combination with DTT or Tm. Cleavage of both caspase-3 and -7 was greater in both HeLa and MAC-T cells when they were treated with RTA and DTT or RTA and Tm compared to cells treated with RTA, DTT, or Tm alone. These results indicate that RTA is more cytotoxic when UPR is inhibited.