The peripheral blood lymphocyte count has been measured in 74 cases of histologically proven carcinoma of the gastrointestinal tract. The count has been correlated with the pathological stage of tumour spread and the patient's delayed hypersensitivity response to 2.4 dinitrochlorobenzene (DNCB). A statistically significant correlation was found between the peripheral blood lymphocyte count and the response to DNCB. There was linear association between the extent of spread of the tumours and the lymphocyte count. Those patients with low peripheral blood lymphocyte counts tended to have more advanced tumours and a poor response to DNCB. The possible causes of this lymphopenia are discussed.