“…Thus, in chickens, feeding of penicillin causes no change in the spleen weight but produces a decrease in weight and lymphocyte concentration of the ileocaecal tonsil and conversely an increase in weight and cell con tent of the thymus [G ordon el al., 1958], The same authors also reported that several other antibiotics, at various dosage levels, did not produce as uniform and pronounced changes as penicillin. According to Braude et al [1955], feeding pigs with tetracyclines does not alter the spleen weight, whereas, significant differences have been noted by others [Taylor and H arrington, 1955;Barber et al, 1957]. Similarly, penicillin has been reported to cause an increase in the blood white cell counts of poultry [Glick, 1958], but in calves treated with chlortetracycline, the white cell count decreases [R ussoff et al, 1964], O wen and A llen [1964] and P ritchard et al [1964], found no change in the total or differential leu cocyte counts in calves on antibiotic-supplemented diets.…”