Commercially produced haem fraction of abattoir blood with a dry matter content of 35% was treated with formic acid of 80–85% purity at the rate of 3:100 (v/v) which transformed the “haem” from a liquid to a solid state and reduced pH from 7.2 to 3.6. The product was included at approximately 5% of the dietary dry matter for pigs over the live weight range 33–80 kg. In one experiment the diet containing the haem was compared with diets containing either 7 or 10% herring meal on an air dry basis. The pigs on the 10% herring meal diet grew slightly, but non-significantly, faster and more efficiently than pigs on the other two diets. The carcases of the pigs on the haem diet contained less lean and more fat than those on the herring meal diets, apparently because of dietary deficiencies of isoleucine and methionine + cystine. In the second experiment, diets with and without the haem fraction were formulated by linear programme taking ingredient costs into account. The performance and carcase composition of pigs on the haem diet were equal to or better than those on the control diet. The total lysine content of the haem fraction of abattoir blood treated with formic acid was 115 g/kg dry matter with an availability (1-fluoro-2,4-dinitrobenzene-reactive lysine) of 86%. In relation to the requirements of the finishing pig the limiting amino acids appear to be isoleucine and methionine + cystine.