Abstract
Ampicillin was included (25 mg/litre) in the drinking water supplied to growing rats over periods of 2, 3, 4, 5, 6, 7 or 8 weeks. 32 rats were used in each experimental period, half being given the antibiotic and half being left as controls. The antibiotic increased the liveweight gain in each period, the results being significant after the 2- and 4-week feeding periods. Significant decreases were observed in the average percentage weights of the spleens of the rats given the antibiotic at the end of all the feeding periods except one. The few other significant changes in the percentage weights of other organs were, with one exception, noted at or before the end of the 4th week.