Male Wag rats were pretreated for 7 days with 1000 U/ml recombinant murine granulocyte macrophage colony stimulating factor (rmGM-CSF). Rat Kupffer cells (KC) were isolated by a enzymatic method. We injected decreasing numbers of CC531 tumor cells in the portal system. Mean KC yield increased from 1.5 +/- 0.2 to 2.2 +/- 0.2 (p < 0.05). Mean percentage of KC-mediated cytotoxicity against CC531 increased from 20.0 +/- 0.5 to 42 +/- 1.0 after rmGM-CSF (p < 0.05). At 1 x 10(5) CC531 tumor cells we demonstrated prevention of formation of small foci of CC52+ tumor cells. We demonstrate increased isolated KC with enhanced cytotoxic capacity after rmGM-CSF. rmGM-CSF induced prevention of minimal residual disease in the rat liver.