Perez Hernandez, Robinson, Aitken and Fraser (1986) showed that the addition of a protected lipid to the diet of lactating ewes increased the fat content of their milk but had little effect on milk yield. The effects on milk composition were reflected in an improvement of approximately 10 per cent in lamb growth rate. In the experiments of Perez Hernandez et al. the response to the lipid supplement was tested using a diet with a high content of undegraded dietary protein. We now report the results of an experiment in which the response of lactating ewes to a ‘protected’ lipid was tested at three levels of protein intake. We also present data from a second experiment carried out to investigate the influence of the ‘protected’ lipid on the fatty acid composition of the ewe's milk.