We have determined that when an extract of human brain is preincubated with lymphocytes its subsequent capacity to inhibit the electrophoretic mobility of tanned and stabilized erythrocytes is much reduced. There is a differential effect, however, as the observed reduction is from 73% inhibition to approximately 35% when the pre-incubation is with lymphocytes from patients with malignant disease, but from 73% to approximately 10% when it is with lymphocytes from normal controls. These values were obtained at a brain extract concentration of 333 microgram/ml, with 5 times 10(6) lymphocytes, a pre-incubation time of 18-24 h, and a temperature of 27 degrees C, which are the optimum conditions determined for differentiation between cancer patients and normal subjects. In a series of 73 subjects tested by this method 43/51 cancer patients gave an unequivocal "positive" value, 22/22 normal controls gave a "negative" value, with no false positives.