Bag1 proteins bind heat shock protein M(r) 70,000 (Hsp 70) family molecular chaperones and regulate diverse pathways involved in cell proliferation, apoptosis, and stress responses. Four isoforms of Bag1 can be produced from a single gene in humans, including a nuclear-targeted long version (Bag1L)and a shorter cytosolic isoform (Bag1). Because overexpression of Bag1and Bag1L has been reported in breast cancers, we explored the effects of Bag1 and Bag1L on the growth of ZR-75-1 human breast cancer cells cultured in vitro and in tumor xenograft models using immunocompromised mice. Cells stably transfected with expression plasmids encoding either Bag1 or Bag1L displayed comparable rates of growth in cultures containing 10% serum, compared with control-transfected ZR-75-1 cells. In contrast, ZR-75-1 cells stably expressing mutants of Bag1 or Bag1L, which lack the COOH-terminal domain (DeltaC) required for heat shock protein M(r) 70,000 binding, displayed retarded growth rates. When cultured without serum, the viability of control-transfected, as well as Bag1DeltaC- and Bag1LDeltaC-expressing, cells declined with time, whereas Bag1- and Bag1L-overexpressing ZR-75-1 cells survived for over a week in culture. Caspase protease activation induced by serum deprivation was also prevented by stable expression of either Bag1 or Bag1L in ZR-75-1 cells. In addition, sensitivity to anchorage dependence was restored partially in ZR-75-1 cells expressing dominant-negative Bag1DeltaC and Bag1LDeltaC. In tumor xenograft studies involving injection of ZR-75-1 cells into mammary fat pads of female nu/nu mice, ZR-75-1 cells expressing Bag1 or Bag1L formed 1.4-1.6-fold larger tumors compared with control-transfected cells, whereas tumors formed by Bag1DeltaC- and Bag1LDeltaC-expressing cells grew very slowly and reached sizes < one-third of tumors generated by Neo-control ZR-75-1 cells. Altogether, these findings demonstrate that Bag1 and Bag1L provoke similar changes in breast cancer cell growth and survival and suggest that interference with Bag1 or Bag1L function might be a useful strategy for opposing breast cancer.