Whilst it is well recognised that there is considerable within-breed variation in growth performance and body composition of cattle it is unusual for sire origin to be taken into account when designing nutritional studies. In planning an experiment on the effects of plane of nutrition and slaughter weight on the body composition and energy metabolism of Friesian steers it was recognised that the increased incidence of the Holstein gene pool in many AI bulls could have serious implications for interpretation of the results. Hence AI bulls of low Holstein content were identified and an attempt was made to obtain calves from a small number of bulls of uniform conformation.