Surgical operations depress immune responses. The aim of this study was to observe the effects of operation (amputation) on the growth of the Lewis lung tumour and its metastases and to see if C. parvum counteracted them. Anaesthesia alone, or with amputation, did not affect the growth of the primary tumour but C. parvum depressed it. Anaesthesia did not affect the number of pulmonary metastases but amputation caused significant increase. C. parvum inhibited metastases and completely counteracted the effects of operation. Large amounts of cortisone significantly increased metastases but small doses did not. Experiments in adrenalectomized mice suggested the effects of operation were due to non specific stress.