Abstract: A method of inhibiting the development of malignant tumors and the formation of metastases of malignant tumor cells in a subject carrying the comprises administering to the subject IL-2 and an agent selected from the group consisting of histamine, a histamine structural analogs having H.sub.2 -receptor activities, an endogenous histamine releasing preparation, and a non-histamine derivative H.sub.2 -receptor agonist; the agent and the IL-2 being administered in amounts and for a time effective to attain the desired effect. A method of increasing the anti-tumor cell effect of IL-2 in a subject comprises co-administering to the subject IL-2 and an agent such as histamine, histamine structural analogs having H.sub.2 -receptor activities, endogenous histamine releasing preparations or H.sub.2 -receptor agonists, the agent and the IL-2 being administered in anti-tumor effective amounts and for a period of time effective to attain the desired effect.