Cytoreductive chemotherapy and immunosuppression have been postulated as possible adjuncts to cancer immunotherapy in studies using murine tumour-infiltrating lymphocytes (TIL). Treatment of animals with cyclophosphamide (Cy) therapy alone caused two distinct biological activities that altered the relationship between host and tumour. These two in vivo activities were distinguished by altering the timing and dose of Cy administration relative to tumour implantation. Cy administered 3 days following tumour injection caused a significant decline in the number of pulmonary micrometastases and greater survival compared to untreated controls in proportion to the dose of Cy administered. Further reduction in pulmonary disease was observed when Cy-treated mice were given TIL therapy. The possible role(s) of Cy-induced immunosuppression was studied by injecting Cy 24 h prior to tumour injection. This treatment failed to cause the cytoreductive effect observed when Cy was administered 3 days after tumour since Cy-administration prior to tumour resulted in a significantly higher number of pulmonary metastases and diminished survival compared to untreated controls. Despite the increased number of pulmonary metastases and decreased survival in mice treated with Cy before administration of tumour, therapy with TIL significantly diminished pulmonary disease compared to animals treated with Cy alone. Immunosuppression (without concomitant cytoreductive therapy) prior to TIL treatment significantly prolonged survival. Additional studies with TIL therapy indicate that the survival of animals immunosuppressed prior to tumour injection was significantly longer than controls which received immunotherapy alone. These results suggest that the combustion of immunosuppression plus cellular immunotherapy, which leads to significant survival advantage in these murine tumour models, may possibly augment the clinical response in human TIL trials.