The overall efficiency of nitrogen utilisation by ruminants is variable, but often very low. The efficiency of capture of degraded nitrogen by rumen microbes is an important determinant of the overall efficiency of utilisation of nitrogen by dairy cows (Dewhurst et al, 1996). Existing marker-cannula techniques for assessing rumen microbial efficiency, and their derived models, do not provide a useful description of the situation on individual farms. There is considerable scope for and improvement in the efficiency of utilisation of nitrogen within dairy systems if a simple diagnostic test of rumen function could be applied. It is envisaged that such a test would contribute to more refined use of protein supplements. A considerable amount of work (see review by Stangassinger et al, 1995) has shown that urinary allantoin excretion is a good index of microbial yield which closely correlates with duodenal flow of purine bases (which originate largely from rumen microbes). However, urine samples are not easy to obtain in the practical situation and so this work investigated the possibility of using allantoin in milk (which is easily sampled) as a microbial index by comparing it with urinary allantoin (the existing technique) and milk protein yield (a product of microbial protein).