Summary The dose response curves of purified normal and chronic lymphocytic leukaemic lymphocytes to highly purified phytohaemagglutinin in 3‐ and 6‐day cultures have been defined. Normal lymphocytes cultured for 3 days were maximally stimulated by low concentrations of phytohaemagglutinin, in the range 0·4 to 1·4 μg phyto‐haemagglutinin/106 cells/ml. Higher concentrations of phytohaemagglutinin caused no further stimulation. Normal lymphocytes cultured for 6 days showed a progressively greater response to increasing concentrations of phytohaemagglutinin up to 15 μg phytohaemagglutinin/106 cells/ml. Chronic lymphocytic leukaemic lymphocytes responded subnormally to all ranges of phytohaemagglutinin concentrations tested, in both 3‐ and 6‐day cultures. The shape of the dose response curve resembled the normal curve in 2 of 5 cases of chronic lymphocytic leukaemic lymphocytes studied in 3‐day cultures, and in all 7 cases studied in 6‐day cultures. No evidence could be adduced of a consistently greater response of chronic lymphocytic leukaemic lymphocytes in 6‐day compared with 3‐day cultures. Five hours’ exposure of normal lymphocytes to an optimum phytohaemagglutinin dose did not result in maximum H3‐thymidine incorporation after 3 days in culture.