The ability of liposomes containing a synthetic lipophilic muramyl dipeptide derivative, N-acetylmuramyl-L-alanyl-D-isoglutamyl-sn-glycerol dipalmitate (MDP-GDP), to inhibit the growth of experimental B16-F1 melanoma liver metastases in syngeneic C57BL/6 mice has been determined. Multiple i.v. injections of distearoylphosphatidylcholine:dimyristoylphosphatidylglycerol liposomes (1 mumol, 10:1 molar ratio) containing 0.1 to 1 microgram of MDP-GDP given at 3- to 4-day intervals after seeding the livers with tumor cells resulted in a significant inhibition of the number of experimental B16 liver metastases. Control liposomes or free MDP (100 micrograms) failed to affect the number of experimental metastases. A single prophylactic injection of liposomes containing MDP-GDP was equally effective in eliciting a reduction in the number of experimental liver metastases. The ability of liposomal MDP-GDP to inhibit the growth of liver metastases correlated with its ability to induce Kupffer cell tumoricidal activity against the tumor cell targets; activation of C57BL/6 Kupffer cell activity in vitro was most effective with liposomal MDP-GDP, followed by liposomal MDP and free MDP. Only liposomal MDP-GDP and liposomal MDP were able to induce Kupffer cell tumoricidal activity in situ, free MDP being inactive. Liposomal muramyl dipeptide therapy using lipophilic derivatives would appear to be an effective treatment for hepatic metastases derived from primary tumors.