Sulfasalazine (SASP) is a novel, potent inhibitor of cellular cystine uptake mediated by the xc−cystine/glutamate antiporter. Lymphoid cells cannot synthesize cyst(e)ine and depend for growth on its uptake from their micro-environment. We previously showed that SASP (0.2 mM) can abrogate lymphoma cell proliferationin vitroby specifically inhibiting xc−-mediated cystine uptake. Intraperitoneal administration of SASP to Noble rats markedly suppressed Nb2-U17 rat lymphoma transplant growth, notably without major toxicity to the hosts. Since Nb2-U17 cells are xc−-deficient, the growth arrest was apparently not due to SASP–tumor cell interaction, but possibly to interference with xc−-mediated cysteine secretion by somatic cells. In this study we found that replication of xc−-deficient Nb2-11 lymphoma cells can be sustainedin vitro, in the absence of cystine uptake enhancers, by co-culturing with IMR-90 fibroblasts known to secrete cysteine. SASP, at 0.15 and 0.2 mM, arrested replication of fibroblast-driven Nb2-11 cells by 93 and 100%, respectively, without impeding fibroblast proliferation. Addition of 2-mercapto-ethanol (60 μM), a cystine uptake enhancer, almost completely prevented this growth arrest, indicating that SASP specifically inhibited cysteine secretion by the fibroblasts, a process based on xc−-mediated cystine uptake. It is proposed that the lymphoma growth-inhibitory activity of SASPin vivoinvolves inhibition of cysteine secretion by tumor-associated somatic cells (macrophages, dendritic cells), leading to cysteine starvation of the tumor cells and apoptosis. The difference between the lymphoma cells and fibroblasts in sensitivity to SASP treatment is consistent with the marked antitumor effect of SASP lacking significant side effects.