SUMMARYThe i.v. injection of 0.3 ml of phytohemagglutinin into mice led to an increase in the number of spleen colony-forming units (i.e., hematopoietic stem cells) circulating in the blood 72 hr later. There was also some small increase in the ability of lymph node cells from similarly treated mice to form spleen colonies after injection into irradiated syngeneic recipients. Of 14 lymph node-derived colonies 11 were erythropoietic and 3 granulopoietic. No “lymphopoietic” colonies were observed, although lymph node cells from both phytohemagglutinin- treated and normal mice repopulated the spleens of the irradiated recipients in a more diffuse fashion. The other detected sequelae of phytohemagglutinin injection were extensive hepatic necrosis in more than half the recipients and a slight leukopenia. Cardiac blood drawn from normal mice in the afternoon contained significantly fewer leukocytes and colony-forming units than blood drawn in the morning. No evidence was obtained in favour of the view that spatially circumscribed, separable clones of lymphoid cells can develop in the recipients of lymphocytes from phytohemagglutinintreated donors.