It has been established in preclinical models of multiple myeloma and acute myeloid leukemia (AML) that the bone marrow microenvironment provides protection from chemotherapy-and death receptor^mediated apoptosis. This form of resistance, termed de novo drug resistance, occurs independent of chronic exposure to cancer-related therapies and likely promotes the development of multidrug resistance. Consequently, it is of major interest to identify compounds or drug combinations that can overcome environment-mediated resistance. In this study, we investigated the activity of tipifarnib (Zarnestra, formerly R115777) combined with bortezomib (Velcade, formerly PS-341) in microenvironment models of multiple myeloma and AML. The combination proved to be synergistic in multiple myeloma and AML cell lines treated in suspension culture. Even in tumor cells relatively resistant to tipifarnib, combined activity was maintained. Tipifarnib and bortezomib were also effective when multiple myeloma and AML cells were adhered to fibronectin, providing evidence that the combination overcomes cell adhesion^mediated drug resistance (CAM-DR). Of importance, activation of the endoplasmic reticulum stress response was enhanced and correlated with apoptosis and reversal of CAM-DR. Multiple myeloma and AML cells cocultured with bone marrow stromal cells also remained sensitive, although stromal-adhered tumor cells were partially protected (relative to cells in suspension or fibronectin adhered). Evaluation of the combination using a transwell apparatus revealed that stromal cells produce a protective soluble factor. Investigations are under way to identify the cytokines and/or growth factors involved. In summary, our study provides the preclinical rationale for trials testing the tipifarnib and bortezomib combination in patients with multiple myeloma and AML.