The purpose of this study was to establish the normal range of serum apolipoprotein B-100 (APO B-100) concentration in clinically normal cattle, and to assess its abnormalities with clinical diseases. We measured the serum concentration of APO B-100 in cattle of varying ages, breeds and sex, maintained under normal field conditions. Blood samples were obtained from 735 apparently healthy cattle and 146 cows with various diseases. The concentration of serum APO B-100 in cattle was assayed by the single radial immunodiffusion method. The concentration of serum APO B-100 in healthy adult breeding bulls (mean ± SD: Holstein; 101 ± 46 μg/ml, Japanese Black; 106 ± 46 μg/ml) was significantly (P<0.001) lower than that in cows (Holstein; 259 ± 63, Japanese Black; 210 ± 46 μg/ml), while that of APO B-100 in steers (Holstein; 290 ± 86 μg/ml, Japanese Black; 302 ± 90 μg/ml) was similar to the level in cows. The concentration of serum APO B-100 in cattle varied with sex and breed. APO B-100 concentration in cattle was decreased in association with metabolic disorders such as ketosis, displaced abomasum and fatty liver. From these results, it is assumed that the level of serum APO B-100 will be applied to diagnosis of metabolic diseases in cattle.