The tumour-host relation is of extreme complexity; moreover, it may change during tumour progression. Information regarding this relation may be of importance in appreciating the efficiency of immunotherapy. In the present study, the effect of splenectomy on tumour growth in two murine tumour progression models, the Lewis lung carcinoma and AKR lymphoma, was examined. The effect of spleen ablation on growth of cells derived from primary and metastatic tumour cells was tested. The data obtained showed a differential effect of splenectomy on the growth of primary versus metastatic tumour cells in the two tumour systems, indicating a differential host response induced by the two cell types and/or a differential tumour cell sensitivity to immune reactions. Surprisingly, the spleen appeared to have a defensive role against metastatic tumour cells and a growth enhancing influence on primary tumour cells. The instability of the tumour-host relation may have important implications for the chances of immunotherapy to serve as an efficient cancer treatment.