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The in vitro responses of lymphoid cells from the blood of children with acute lymphatic leukaemia to stimulation with phytohaemagglutinin, pokeweed mitogen, allogeneic lymphocytes and a cocktail of five different antigens were studied. In addition the stimulatory capacity in mixed lymphocyte culture was investigated as well as the formation of rosettes with sheep erythrocytes which were either uncoated or coated with antibody and complement. Whereas lymphocytes from acute lymphatic leukaemia patients in remission responded well to all stimuli tested, lymphoid cells from all seven patients in leukaemic stages with at least 80% lymphatic cells in the differential leucocyte count uniformly displayed strongly reduced or even absent responses. The phytohaemagglutinin response of cells from leukaemic stages was not only reduced but also delayed and the stimulatory capacity in mixed lymphocyte culture was always stronger than the responding capacity. These data indicate that the responses of cells from leukaemic stages are due to the presence of a small proportion of residual normal lymphocytes whereas the leukaemic cells themselves are unable to respond. The stimulatory capacity in mixed lymphocyte culture was reduced in four cases. The cells from two patients, however, displayed increased stimulatory capacity. The cells from different patients varied strongly with respect to the presence of surface markers as tested by rosette formation with sheep erythrocytes either uncoated or coated with antibody and complement. [ABSTRACT FROM AUTHOR]