Proteins which escape digestion and absorption in the small intestine of the pig are extensively metabolized by the microbial flora of the large intestine. It is known that bacterial enzymes are potentially capable of catabolizing all amino acids (Michel, 1966). Furthermore, it has been shown that virtually all soluble protein or amino acids introduced into the caecum are rapidly metabolized, absorbed (presumably as ammonia, amines etc.) and the nitrogen (N) derived from them is rapidly excreted as urea (Zebrowska, 1973). This has led to the general recognition that the amino acid composition is not a reliable measure of undigested dietary amino acids in pigs. It has been estimated that only 6% of faecal N is of undigested dietary origin, while the remainder is of bacterial or endogenous origin (Low, Sambrook & Yoshimoto, 1978). It was therefore of interest to estimate which proportions of faecal N derived from a barley-soya diet, are of either bacterial, endogenous or undigested dietary origin. We have used germ-free or conventional miniature pigs for this work because they are small enough to be kept in our isolators for periods of up to 20 weeks. It is emphasised that this is a preliminary report on our first efforts at raising miniature pigs under germ-free conditions.