The fat composition of the diet fed to C57BL/6J mice influenced the effect of Levamisol on the growth and spread of LLT. The timing of feeding of different diets as well as of the administration of Levamisol in relation to tumor transplantation was of importance. The tumor-suppressive effect of Levamisol given on day 9 was counteracted by a diet containing 2% soy oil instead of animal fat and given from day 1. Levamisol given on day 1 counteracted the facilitation of tumor growth and spread by the soy oil diet given from day 9. The soy oil diet was shown to suppress the immune response to SRBC. We conclude that standardization of the laboratory diets is necessary.