During work on the effect of penicillin on vitamin A metabolism in the chick (Coates, Harrison, Kon, Porter & Thompson, 1952) it was observed that the small intestines of birds given penicillin in the diet appeared thinner than those of their controls. A further investigation was therefore undertaken to study the importance of this phenomenon in the mechanism of antibiotic growth promotion.The experiments were of two types, the first dealing with the quantitative and qualitative changes in the gut and the second with the possible mode of action of penicillin in reducing gut weight. They have accordingly been presented here in two separate sections.

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Unsweetened evaporated milk is widely used in infant feeding. This product is subjected to drastic heat treatment during sterilization in the can. Lately there have been several reports (l, 2, 3, 4, 5, 6) that in the process of manufacture evaporated milk loses a large part of its original vitamin B 6 activity. According to Tomarelli, Spence & Bernhart(5) the loss of vitamin B 6 , as judged microbiologically, is much less than that determined with rats. We present here the results of a reinvestigation of the problem. The vitamin B 6 activities of raw, evaporated and stored evaporated milk were measured with rats, and chicks, and also microbiologically. In addition, the activities of the pure analogues for chicks and rats were determined to give a basis for comparison. The microbiological techniques have been separately studied by one of us and the findings are set out in the accompanying paper (7). EXPERIMENTAL MaterialsFrom a 36 gal. bulk of fresh mixed full cream milk from the N.I.R.D. herd, 18 gal. were freeze-dried immediately. The other 18 gal. were evaporated, canned and sterilized as described in (8). Half was freeze-dried immediately and half after 6 months storage at room temperature. The freeze-dried milks were stored at -15° C. in air-tight containers until required.Solutions of the pure vitamins, pyridoxine hydrochloride and pyridoxamine dihydrochloride (Roche Products Ltd.) and pyridoxal hydrochloride (L. Light and Co.) were made to contain 500 fig. free base/ml, and used as described below. Chick assaysThe method developed by Coates, Kon & Shepheard (9) was used. The basal diet given in Table 1 contains, with the exception of vitamin B 6 , all characterized vitamins believed to be essential for the chick. With vitamin B 6 added in sufficient quantities it supports excellent growth. As prepared, the diet is a fine powder not readily eaten by chicks and past experience has shown that it is much better granulated.The freeze-dried milk samples were incorporated into the diet to form 25,12-5 or 6-25% of it, replacing an equal weight of casein and dextrin (in the ratio 3:2). These quantities were too large to be given separately by mouth. For this reason the pure analogues were tested in the same way, being included during granulation at the rate of 250,125, 62-5 or