Microbial exopolysaccharides (EPSs) are highly heterogeneous polymers produced by fungi and bacteria and have recently been attracting considerable attention from biotechnologists because of their potential applications in many fields, including biomedicine. We have screened the antitumoural activity of a panel of sulphated EPSs produced by a newly discovered species of halophilic bacteria. We found that the novel halophilic bacterium Halomonas stenophila strain B100 produced a heteropolysaccharide that, when oversulphated, exerted antitumoural activity on T cell lines deriving from acute lymphoblastic leukaemia (ALL). Only tumour cells were susceptible to apoptosis induced by the sulphated EPS (B100S), whilst primary T cells were resistant. Moreover, freshly isolated primary cells from the blood of patients with ALL were also susceptible to B100S-induced apoptosis. The newly discovered B100S is therefore the first bacterial EPS that has been demonstrated to exert a potent and selective pro-apoptotic effect on T leukaemia cells, and thus, we propose that the search for new antineoplastic drugs should include the screening of other bacterial EPSs, particularly those isolated from halophiles.