Thirty Holstein cows in early lactation were used in a randomised complete block design to compare a lysine deficient diet, which was sufficient in methionine, to the same diet supplemented with a rumen protected lysine product. The lysine supplementation resulted in an optimal dietary lysine : methionine ratio in metabolisable protein of 7.2:2.4. Lysine supplementation did not affect dry matter intake, milk production, milk fat %, milk protein %, milk urea nitrogen, body weight or body condition score but decreased the non casein nitrogen and whey content of milk. Furthermore, milk casein, which is the milk nitrogen fraction most sensitive towards increased duodenal supply of lysine and methionine, was not affected. The rumen protected lysine product evaluated did not improve cow productivity, probably because the product was not protected sufficiently.