SUMMARYFive gnotobiotic lambs were fed on sterile diets until they were killed at 13 to 2 1 weeks of age. They were dosed orally with different combinations of I I species of rumen bacteria. The biochemical reactions of each of the bacteria inoculated had been determined in pure culture in vitro, and they were chosen to perform the main reactions known to be associated with digestion in the normal mature rumen.Two of the bacteria could not be reisolated, but the remainder had established readily in the rumen, forming stable, mixed, defined populations. The total numbers of bacteria in the rumen, and the viable counts of most of the individual species were comparable to those of normal sheep. The concentration of volatile fatty acids was lower, however, and in four of the lambs there was a higher proportion of butyric acid and a lower proportion of propionic acid than in normal sheep. Cellulolytic, ureolytic, and methanogenic activities appeared to be taking place and lactate-utilizing bacteria appeared to reverse the accumulation of lactate which resulted from the activity of lactate-producing bacteria.Some of the bacteria also established at high levels in the caecum. I N T R O D U C T I O NThe rearing of small omnivorous and carnivorous gnotobiotic animals is a well-established laboratory procedure (Coates, 1 9 6 8 ) but ruminants present particular difficulties. Digestion in omnivorous or carnivorous animals can take place without the intervention of gut micro-organisms, but bacteria are essential for digestion in mature ruminants. A young ruminant, fed on milk, might be expected to behave as a monogastric animal and to thrive in the gnotobiotic state while its rumen is still undeveloped, but in the mature ruminant the main digestive action is provided by the micro-organisms of the rumen, regardless of the form of the diet.The principal reactions of the main components of the rumen microflora, reviewed by Hungate (1966) andHoward (1969), have been established largely from studies of pure cultures in vitro. Information on interactions between rumen bacteria has been obtained from mixed cultures which rarely comprise more than two bacterial species. In such studies 'animal factors are missing and rumen conditions cannot be reproduced. In vivo studies in normal mature ruminants are complicated by the great variety and * Present address : University of Edinburgh, Veterinary Field Station, Easter Bush, Roslin, Midlothian, Scotland.

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SUMMARYSeventeen gnotobiotic lambs were reared up to 21 weeks of age on cows’ milk followed by sterile solid diets similar to diets fed to conventional lambs. Seven were given limited defined populations of rumen bacteria, seven were left uninoculated and three were dosed with rumen contents from conventional sheep (‘conventionalized’). The lambs were reared in four groups corresponding to four lambing seasons.Gnotobiotic lambs fed cows’ milk ad libitumgrew at similar rates and converted feed into live-weight gain with similar efficiency to conventional lambs. The gnotobiotic lambs irrespective of inoculation and the two conventionalized lambs in the first three groups failed to maintain growth on solid diets when milk feeding had been stopped. However, three gnotobiotic lambs in the fourth group, two of which were inoculated, continued to grow satisfactorily for at least 6 weeks on a solid diet alone, and the inoculation of defined populations of bacteria appeared to confer an advantage. The conventionalized lamb in the fourth group suffered a severe temporary setback, following which it grew well.

This paper discusses the contaminants that were detected in gnotobiotic lambs reared for up to 5 months.