gm./ton (approximately 200 p.p.m.) not every chick had detectable amounts of the anti- 
biotic in their blood. One day after cessation of antibiotic feeding the serum contained no 
antibiotic when the feeding levels had been 200to 1,000 gm./ton (approximately 200-1,000 
p.p.m.). When the dietary level was 5,000 gm./ton, some was found in the serum until 
after the third day (18). 
The antibiotic was found in the liver when the diet contained 50 p.p.m. chlortet- 
racycline or 200 gm. oxytetracycline per ton and in the muscle at dietary levels of 
50 p.p.m. and 1,000 gm./ton, respectively. The tissue content actually decreased as 
time on the diet increased (18,20). These differences noted may be due to absorption, 
assay method, or experimental technique. It disappeared rapidly from the tissues when 
the antibiotic was withdrawn from the diet. The levels in the liver and muscle were 
practically zero one day after cessation of feeding levels of 1,000 or 2,500 gm. oxytet- 
racycline/ton feed. At higher levels it persisted in the kidney and intestine for about 3 
days. Twenty-four hours after cessation of feeding 200 p.p.m. chlortetracycline in the 
diet 94 percent of the antibiotic in the muscle, liver, and blood had been excreted 
(6,18,20,21). 
Chlortetracycline feeding at the rate of 200 p.p.m. to laying hens produced low 
amounts (0 to 0.1 Mg./gm.) of the antibiotic in the eggs. Here also the concentration in 
the eggs decreased with increasing length of time that the antibiotic was fed at this level 
as well as at the 2,000 p.p.m. level of feeding. After withdrawal for 1 week none was 
found in the eggs when the feeding level was 200 p.p.m. and only small amounts (0 to 0.13 
ug./gm.) persisted for 2 weeks at the 2,000 p.p.m. feeding level (18). 
The adjuvants also increase the concentration of chlortetracycline in the tissues, 
but no information is available on the amount in eggs. 
The usual feeding level to promote growth in poultry is 4 to 10 gm. antibiotic per 
ton of feed and at this level no antibiotic has been found in the blood serum or body 
tissues of chickens. At disease prevention or treatment levels of 50 to 200 gm./ton the 
antibiotic can be found in serum and body tissues but persists for only about 1 day after 
withdrawal. 
The problem of a residue in poultry and swine is becoming limited where there is an 
increased tendency to use bacitracin or streptomycin that are not absorbed from the G.I. 
tract, or penicillin that is rapidly and quantitatively excreted. 
Chlortetracycline and tetracycline appear in the bone and bone marrow after oral or 
parenteral administration. The antibiotic is apparently unaltered and permanently bound 
there in metallic combination in the newly formed bone. These antibiotics in high con- 
centration impart a yellow color to the newly formed bone and smaller quantities can be 
detected by their ultra-violet fluorescence. It can only be eluted from the bone tissue by 
means of weak acid or EDTA. Cycline antibiotics are also bound by peptides in certain 
tumor tissues (23,24,25). 
Metabolism 
Penicillin: Penicillin was found to be bound to serum proteins and was isolated 
electrophoretically with them. The same amount of penicillin per unit of nitrogen was 
found in all protein fractions but fractions containing no nitrogen contained no penicillin. 
Penicillin added to human or guinea pig serum in vitro combines quantitatively with the 
serum proteins (26,27). 
Penicillin labeled with radioactive S35 was given intramuscularly to cats and 100 
percent of the radioactivity was recovered inthe urine but simultaneously only 50 percent 
of the biological activity was so accounted for. When given orally 38 percent of the radio- 
activity was found in the urine within 8 hours while only 20 percent of the biological 
activity was found in the urine. The majority of the radioactivity remained in the G.I. 
tract (28). 
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