The cell mediated immune response has been measured in vitro by lymphocyte transformation in 53 patients with malignant tumours of the breast and an equal number of patients in the same age group operated upon for benign tumours. The response to phytohaemagglutinin (PHA) was reduced in the patients with malignant tumours when the lymphocytes were grown either in the patient's plasma or in autologous plasma. This was observed when the response was measured both by the uptake of radioactive DNA precursor and by autoradiography. Reduced lymphocyte transformation was present even very early in the disease and certainly preoperatively. The reduction appeared to be even more marked in the advanced cases. An interesting finding was that the plasma from cancer patients contained a factor which reduced the PHA transformation of lymphocytes from a healthy donor.