in larger concentrations (12 times larger) in the feces and in all sections of the gastro- 
intestinal tract than did intramuscular injection of the same dose when only very small 
amounts were found in the lower intestine and feces. None was found in the rumen after 
intramuscular administration. Conversely after intramuscular injection, urine levels were 
four to five times that obtained after oral administration. Similarly, liver and kidney had 
measurable amounts but only traces after oral administration. With both methods of ad- 
ministration the bile contained the same concentration (0.6 y4g./ml.), while only traces 
were found in the blood (12). When a single dose of 125 mg. was given, appreciable levels 
were found in the blood, liver, and kidney for 7 days (7). Three days after withdrawal 
following long-term ingestion of about 500 mg./day/calf the liver contained 0.02 ug. 
chlortetracycline/gm. but none was found inthe bloodor kidney (7). Information with other 
species indicates that the liver may act as a place of storage for the cycline antibiotics 
(2,5,18,30). 
One intramuscular injection of a mixture of benzathine, procaine, and potassium 
salts of penicillin G (12,500 to 50,000 units/kg.) to dairy steers produced a maximum 
blood level (2-9 yg./ml.) within 1/2 hour and measurable amounts (0.02 to 0.05 wg./ml.) 
were found in the serum for 6 days (13). 
When 130 to 700 mg. of the tetracyclines were ingested per day by lactating cows, 
the milk produced did not alter the growth of several sensitive cheese starter cultures 
which indicated that very little of the antibiotic was secreted into the milk (14, 15, 16). 
When 0.1 mg. chlortetracycline/lb. body weight (100 mg./day) or when 320 mg. per day 
was ingested, no antibiotic was found in the milk. However, when 0.5 or 1.0 mg./lb. (500 
to over 1,000 mg./day/cow) was ingested, small amounts (0.01 to 0.06 wg./ml.) of the 
antibiotic were found in the milk 24 hours after ingestion. With continued administration 
the concentration of chlortetracycline in the milk increased for a few days to levels of 
0.03 to 0.20yg./ml., and then stabilized at levels of 0.00 to 0.09 wg./ml. No antibiotic 
was found in the milk 24 hours after removal of chlortetracycline from the diet (16,17). 
When fed to dairy cattle, the recommended level is 0.1 mg. chlortetracycline/lb. 
body weight or about 100 mg./cow/day. At this rate none has been detected in the milk by 
several investigators. No information on the content of chlortetracycline in the feces, 
urine, or tissues of these cows is available but based on the reported results with calves 
(7,12) very little, if any, would be expected in tissues of cows receiving the recommended 
level. 
There was no detectable amount of antibiotic in any internal organ or muscle of pigs 
fed the antibiotic from weaning to market weight at the rate of 5 or 83.5 gm. bacitracin/ton 
of feed. The same result applied to penicillin fed at 1 or 16.7 gm./ton of feed or with 
chlortetracycline fed at 10 gm./ton (19). The usual level of feeding to swine is about 10 
gm./ton. 
When oxytetracycline at various levels was continuously fed to pigs, none was found 
in any tissue until the level of 200 gm./ton was reached when low concentrations (0.2 to 
0.7 wg-/gm.) were found in the kidney and intestine, respectively. At levels of 1,000 to 
2,500 gm./ton of feed it appeared in all tissues, but the intestine contained 4 to 5 times 
that found in the liver and kidney (1.0 to 5.0 u4g./gm.). The muscle contained 1.1 to 1.3 
Mg. oxytetracycline/gm. in each case (20). 
Young baby pigs fed a diet containing 200 gm. chlortetracycline per ton of feed had 
this antibiotic in their blood serum at levels of 0.1 to 0.5 wg./ml., but none was present 
after the antibiotic was withdrawn for 2 days (21). 
In none of the above trials is the actual amount of the antibiotic consumed stated. 
Concentration of chlortetracycline was approximately the same in fat as in lean 
muscle tissue (6). 
Chlor-, oxy-, and tetracycline appeared in the blood serum of young chickens when 
fed at the rate of 50 gm. antibiotic per ton of feed. However, even at the level of 200 
207 
