Gallium, a metal with clinical antineoplastic activity, is known to inhibit cellular iron uptake and iron-dependent DNA synthesis. Little information exists regarding the efficacy of gallium in combination with other agents. Since alpha-interferon (IFN-alpha) can modulate the action of certain chemotherapeutic drugs, we examined its influence on the growth inhibitory effects of gallium in CCRF-CEM cells. IFN-alpha and gallium as single agents had only minimal to moderate antiproliferative effects. In combination, however, both drugs synergistically inhibited cell growth, causing cell death accompanied by DNA fragmentation. At lower concentrations (120 microM), gallium inhibited cellular iron uptake but did not increase transferrin receptor expression, nor did it block cellular proliferation. The addition of IFN-alpha to this concentration of gallium significantly increased the gallium-induced block of iron uptake, resulting in an increase in transferrin receptors and an inhibition of cell growth. In contrast, IFN-alpha did not enhance the effects of the iron chelator deferoxamine on iron uptake or cell growth. Our studies suggest that gallium and IFN-alpha synergistically inhibit DNA synthesis through a mechanism that includes inhibition of cellular iron uptake and depletion of intracellular iron below the critical level needed to maintain DNA synthesis.