Intravenous administration of syngeneic spleen cells from normal donors was found to markedly shorten the induction time for casein-associated splenic amyloid diposition in the mouse. The effect of intravenous donor cells seemed purely one of acceleration; it did not provoke amyloid deposition either by itself or in combination with independently ineffective heterologous proteins. The accelerator effect did not depend on the viability of the cell suspension, and after physical disruption of the cells all the accelerator activity seemed localized to the sedimentable fraction of damaged cells, nuclei and coarse debris. It is suggested that the accelerator acts through affecting the function of perifollicular splenic macrophages.