In earlier studies it was shown that if rabbit lymph node cells were incubated with Shigella-trypsin filtrate and transferred to other, recipient, rabbits, agglutinins to Shigella appeared subsequently in the sera of the recipients. However, if leucocytes from the donor rabbits were injected into the recipients at a suitable interval prior to lymph node cell transfer, agglutinins to Shigella did not appear after cell transfer. In the present study, a number of experiments were done which bear on the immunologic character of this suppressive effect on the transferred cells, the "pre-injection effect." X-irradiation of the recipients before the injection of donor leucocytes led to a decrease in the extent of the pre-injection effect, i.e. to the appearance of higher agglutinin liters, in the recipient animals. It was found possible to transfer the pre-injection effect by cells of rabbit lymph nodes draining sites of injection of rabbit leucocytes. No such effect was obtained with heated aliquots of these cell suspensions or with lymph node cells from sheep-erythrocyte-injected donors. It was also possible to transfer the pre-injection effect passively by serum. Suppression of transferred lymph node cells was observed regularly after injection of serum pooled from groups of rabbits which had been injected with leucocytes pooled from the blood of 60 to 70 rabbits. The active material in anti-leucocyte serum could be precipitated in the globulin fraction of the serum and could be removed by absorption with rabbit lymph node cells. In the course of experiments on the passive transfer of the pre-injection effect by antisera of individual rabbits to leucocytes of individual donor rabbits, evidence was obtained of the existence in the sera of some normal rabbits of antibodies to some of the rabbit leucocyte antigens.