Widescale on-farm assessment of microbial protein supply by urinary purine derivative (PD) excretion in dairy cows is restricted by the necessity for a total urine collection. Evaluation of urinary creatinine (c) excretion as a marker of urinary output, indicated the use of spot urine sampling to be unreliable (Shingfield et al 1995a). Further investigations have indicated that, based on mean treatment measurements, milk allantoin excretion and concentration are closely correlated with fermentable metabolisable energy (FME) supply, over a narrow range of milk yields (Shingfield et al 1995b). The current experiment evaluates the potential of milk allantoin excretion and concentration as a index of microbial protein supply over a much wider range of milk yields.