BACKGROUND Earlier reports on animal studies showed inhibition of secondary tumor growth and metastases after cryoablation, probably mediated by an inflammatory response. In this study enhancement of this inflammatory response and its possible additive antitumor effect is evaluated in a mouse tumor model. METHODS Mice received two subcutaneously implanted C--26B tumors on, respectively, day 0 (thigh) and day 7 (flank). The thigh tumor was treated by either cryoablation or resection. In addition the animals received a single dose of lipopolysaccharide (LPS) or anti-IL10 together with, or two days after, surgical treatment. The growth of the flank tumor was followed and plasma levels of IL-1 alpha and TNF-alpha were measured. RESULTS Compared to excision of the primary tumor, cryosurgery clearly induced inhibition of secondary tumor growth while plasma levels of TNF (0.09) and IL-1 (0.06) were significantly elevated after cryosurgery when compared to excision (TNF 0.0, IL-1 0.03; p < 0.01). Administration of LPS two days after cryosurgery did not lead to extra inhibition of secondary tumor growth, even at high doses. Remarkably, dose--response studies with LPS administered two days after treatment showed a high mortality at a dose of 200 micrograms (75%) in the excision group while mortality in the cryo-treated group was 13% (p < 0.02). Mortality was directly related to cytokine levels that were significantly higher in the excision group (TNF 3.60, IL-1 0.30) when compared to the cryo-treated group (TNF 1.0, IL-10.15; p < 0.01). In contrast, when 25 micrograms LPS was given at the same time as treatment of the primary tumor either by cryosurgery or excision, mortality in the cryo-treated group (85%) was higher than in the excision group (14%, p < 0.05). Again mortality was related to post-treatment cytokine levels which now were significantly higher in the cryo-treated animals (TNF 1.30, IL-10.35) than in animals treated by excision (TNF 0.60, IL-10.10; p < 0.01). Administration of anti IL-10 did not lead to extra tumor growth inhibition. CONCLUSIONS These experiments confirm the hypothesis that cryosurgery leads to a systemic inflammatory response. This reaction can lead to the inhibition of tumor growth. Administration of LPS after cryosurgery does not lead to an extra anti-tumor response; animals appear to become endotoxin tolerant. Adversely, when LPS is administered together with cryosurgery, the animals are extremely sensitive to LPS. These findings are in accordance with the clinical observation of cryoshock after cryoablation of liver metastases.