In a previous study (Slade and Miller, unpublished), weaners fed a protein source high in porcine immunoglobulin (Ig) outperformed contemporaries fed an isonutritious and amino acid balanced diet containing an Ig rich bovine protein source (WGC). Diet analysis indicated differences between the protein source contributions to total dietary methionine (Met); 21.5% and 12.5% of 5.2 g/kg total Met respectively for bovine and porcine materials. Hydrolysis of bovine IgG in the proximal and medial thirds of the weaner small intestine is limited (Morel et al. 1995) and Met absorption from the piglet large intestine is insignificant (Darragh et al., 1991). Small intestine failure to degrade bovine Ig would therefore limit dietary Met availability. This limitation may be further confounded by retention of dietary Cys within the disulphide bridges of non-degraded Ig thereby increasing metabolic demand for Met. The objective of this study was therefore to determine the effect of Met supplementation on the performance of weaners fed an isonutritious diets containing the same level WGC.