The aim of the present study was to investigate the effects of oral administration of the insulin-like growth factor-I-rich fraction (IGF-I-RF) from bovine colostral whey on the regulation of blood glucose levels in streptozotocin (STZ)-induced diabetic mice. We obtained a peptide fraction containing IGF-I (10 ng/mg protein) from Holstein colostrum within 24 h after parturition by using ultrafiltration. The blood glucose levels of STZ-induced diabetic mice fed with IGF-I-RF (50 μg/kg per d) were significantly reduced by 11 and 33 % at weeks 2 and 4, respectively (P < 0·05). The body weights of STZ-induced diabetic mice increased following the oral administration of the IGF-I-RF. The kidney weights of STZ-induced diabetic mice decreased significantly (P < 0·05) following the administration of the IGF-I-RF, and the liver weights of STZ-induced diabetic mice decreased significantly (P < 0·05) following the administration of 50 μg/kg per d of the IGF-I-RF. The present results indicate that the IGF-I-RF obtained from Holstein colostrum could be a useful component for an alternative therapeutic modality for the treatment of diabetes in insulin-resistant patients.