The effects of raw sweet lupin (Lupinus angustifolius) meal on the growth and N utilisation of rats were determined in two ad libitum and two restricted-feeding net protein utilisation (NPU) and ยฎve N balance experiments. Sweet lupin seed grown in Western Australia, obtained as meal, either unsupplemented (LMU) or fully supplemented with required amino acids (360 g kg ร1 ) (LMFS), was tested. Rats fed lactalbumin (130 g kg ร1 ) (LACT) were used as positive controls, while rats fed a nonprotein diet (NPC) were used as negative controls. In addition, seed protein, extracted at pH 7.0 with water and insoluble after dialysis at pH 7.0 (LPADI; 124 g kg ร1 ), was also used. The diets contained the same amounts of energy and protein and were supplemented with essential amino acids, vitamins and minerals to target requirements for rats. Inclusion of LPADI in the diet of growing rats caused urinary losses of N, almost all as urea, hypoproteinaemia and increase in body water that resulted in the lowest NPU values, N balance and growth rate as compared with other diets used. These rats developed atrophy of the spleen (low dry weight) and had a comparatively smaller thymus gland than those given raw meals. Furthermore, the LPADI fraction was shown by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) to contain three polypeptides with molecular weights between 30 and 36 kDa which are similar to lectins obtained from Phaseolus vulgaris, Abrus precatorius and Ricinus communis. It is possible that the toxic protein component in the sweet lupin, which has negligible in vitro haemagglutination properties and is extremely toxic in vivo, exerts toxicity by interfering with protein synthesis in the liver, while the immune responses are secondary to azotaemia (high level of urea in the blood) or cytotoxicity action on lymphocytes. The unusual depletion of fat from the body, however, was due to the failure of absorbed amino acids to assimilate as proteins, creating dietary protein restriction and leading to lipolysis. It is therefore tentatively suggested that sweet lupin seed contains a lectin-like protein that is concentrated in this fraction. Further puriยฎcation and biological evaluation to establish the exact nature of this protein may be important.