ExtractClearance of I125-labelled Type II pneumococci in immune and nonimmune rabbits was determined at 15, 30, 45 and 60 minutes after an injection of 1 × 109organisms (fig. 1). The patterns of uptake by the spleen and liver in immune and nonimmune rabbits differed in several major respects. The overall uptake by the liver exceeded that of the spleen in control and immune animals by a factor of at least 10 at each interval examined, this being most striking in the immune group. The spleen had a consistently higher capacity for uptake per unit of weight than the liver at all time intervals examined. This effect was found in immune as well as in nonimmune animals. The type specificity of this organ uptake pattern of response was determined by challenging an immunized group with a type of organism which differed from the immunogen. The pattern of uptake resembles that of non-immunized rabbits. Studies using passive immunization with type specific and heterologous type antisera confirmed these findings.SpeculationThe results obtained indicate more efficient function of the spleen in nonimmune clearance of these microorganisms but greater activity per unit of weight than the liver in both groups. The liver, although dependent upon type specific antibody for maximal clearance, was found to bear the total greatest burden of both immune and nonimmune clearance. Thus the capacity of the spleen for clearance of single episode of bacteremia may have considerable significance in the absence of specific antibody, in initiating a rapid local specific immune response there.