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When the phagocytic index of the reticuloendothelial system was measured by the rate of carbon clearance from the blood, the clearance rate decreased in older rats( 1) and mice ( 2 ) . I t has also been observed that the Calmette-Gukrin bacillus produced a smaller increase in the phagocytic capacity of older mice as compared to younger mice( 2 ) . Since spontaneous tumors usually appeared in older animals and only a minimal elevation of clearance rates was observed, it seemed possible that this might be partially explained by the inability of the older animal to respond( 2 ) . Following treatment with Mycobacterium phlei the phagocytic index was found to be greater in mice 6 months of age than those 1, 3, 9 or 1 2 months(3).In the experiments with the Jensen tumor the clearance rate of carbon was elevated at an earlier period of tumor growth in the older animals than in the younger tumorbearing rat. This would suggest that the type of stimulus applied may be important in determining the response of the reticuloendothelial system in rats of various ages. The older animals will give a faster rate of clearance than younger animals with some types of stimulation.Materials and methods. Six groups of female Holtzman rats were used in these experiments. The first 5 groups were 2-, 3-, 4-, 5and 6-month-old virgin females at the time of tumor inoculation. The last group of animals were discarded breeders 12 months of age or older. The average initial body weight in the various groups at time of tumor inoculation was 181, 201, 239, 258, 276, and 294 grams, respectively. The tumors were transplanted by an intramuscular injection of the Jensen sarcoma into both rectus fernoris.The activity of the reticuloendothelial sys-tem was determined by slight modifications of the method of Biozzi et aZ(4). A suspension of carbon (C 11/143a)t was injected into the femoral vein at a dose of 16 mgJ 100 g body weight. The rats were kept under light anesthesia during administration of carbon and sampling of the blood. At appropriate intervals blood was collected from the tail vein, and a 0.02 ml sample was pipetted into 5 ml of hemoglobin diluent-1 g NaHC03, 501 mg KCN, and 200 mg KxFe (CN)6 diluted to 1 liter. The concentration of carbon was determined by its absorption at 540 mp in a Beckman DU spectrophotometer. The blanks were samples of blood obtained prior to the injection of carbon. Blood carbon concentration decreased according to an exponential function of time expressed as follows: log C1-log C2/TZ -TI = K C1 and C2 were the concentrations of carbon in the blood at times TI and Tz, respectively. K, which expressed the phagocytic activity of the reticuloendothelial system for the injected dose, was called the phagocytic index.I t has been shown that the phagocytic index depends on the weights of liver and spleen(4). This allows the use of a corrected phagocytic index (a) expressed as: m-w WLS a=---

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