A diet containing 11% per weight fish oils (Max-EPA) reduced the rate of growth of a transplantable human breast tumor (MX-1) grafted into immunodeficient mice (BALB/c, nu/nu) when compared to MX-1 tumors in mice fed polyunsaturated (corn oil) and saturated (lard) fatty acids; however, there was no difference between the corn oil and lard animal groups. Treatment with a cocktail of monoclonal antibodies (MoAbs) decreased the rat of growth of MX-1 in the corn oil fed animals but not in those fed lard or Max-EPA, but the rate of tumor growth of the Max-EPA treated group, either MoAb treated or not, was slower than that of the corn oil and lard groups.