We have previously reported on the efficacy of intraperitoneal (i.p.) immunotherapy with interleukin-2 (IL-2) and adoptively transferred lymphokine activated killer (LAK) cells in an i.p. murine tumour model. Because of a dose-limiting toxicity associated with IL-2, cures are seldom observed. The development of treatment strategies that combine components that augment or synergise with the antitumour activity of IL-2 is crucial for the successful use of IL-2 in a clinical setting. Because of the known toxicity of high-dose IL-2 or high dose cyclophosphamide (CY) treatment, the goal of our experiments was to investigate the efficacy of chemoimmunotherapy with low or moderate doses of cyclophosphamide (CY) in combination with low or moderate doses of IL-2 with or without adoptively transferred LAK cells. Assessment of i.p. tumour growth 14 days after tumour inoculation, using the peritoneal cancer index (PCI) scoring system, demonstrated that combination treatment of established (day 3) i.p. tumour was clearly superior to single modality treatment. The effect was further enhanced by a second dose of CY at the end of a course of IL-2. Combination treatment led to a significant survival benefit. About 25% of the mice were cured, even when the dose of tumour cells at inoculation was increased. These experiments demonstrate the efficacy of combined treatment with IL-2, LAK cells and CY. Further research should be directed at the design of treatment schedules based on repetitive courses of chemoimmunotherapy associated with little toxicity.