Although Chediak-Higashi syndrome (CHS) has been found in various mammalian species, it has not been described in rats. Since giant granules characterizing CHS are easily recognizable in mast cells of beige (CHS) mice, we screened mast-cell granules in the auricle of some mutant rats, of which coat color was diluted by mutation. Giant granules of mast cells were found in a mutant trait that occurred in the inbred colony of the DA strain rat maintained in Hamamatsu University School of Medicine. Giant granules were also observed in neutrophils and pigment cells of the eye. In this mutant, either spontaneous migration or chemotaxis of neutrophils was impaired, and the bleeding time was prolonged. Blood serotonin level of the mutant was about one tenth that of the normal congenic rat, and injection of serotonin normalized the bleeding time of the mutant. Moreover, the natural killer activity of the mutant was significantly impaired. These results indicated that this mutation was comparable to CHS of humans and mice, and we designated it as “beige.” Since rats are more favorable than mice for some types of experiments, the beige rat is potentially useful as an animal model of CHS.