Three experiments were carried out to examine responses of milk production to the intravenous infusion of amino acids in dairy cows given diets of grass silage and supplements based on barley, with or without added soyabean meal and ranging in crude protein content from 16 to 19% in dry matter. Particular attention was given to histidine, administered alone or in combination with methionine, lysine and tryptophan. Responses of milk protein secretion to infusion of histidine were seen only when the diet contained a supplement of barley alone. When soyabean meal was included, there were no responses of milk production to infusion of any of the infused amino acids. Calculations suggested that, although histidine remained first-limiting when soya was included in the diet, any response to infusion of histidine was blocked by the rapidly emerging deficiency of another amino acid, probably leucine. The results confirm that, for diets based on grass silage and supplements of cereal only, histidine is first-limiting such that increases of milk protein secretion can be obtained in response to infusion of histidine alone. In assessing the practical significance of this finding, it should be remembered that greater responses in the yield of milk protein can probably be obtained by substituting 1 kg of soyabean meal for 1 kg of cereal, which is likely to be an easier and cheaper option.