The distribution of S-100 protein in spleens and bursae of Fabricius of broiler chickens with various diseases was investigated by an immunohistochemical method with antiserum to bovine S-100 protein. S-100 protein-positive cells were found in six cases of 34 chickens. In spleens, S-100 protein was detected in reticular cells in the ellipsoids of the sheathed arteries and eosinophilic granulocytes. Follicular dendritic cells in germinal centres were also S-100 protein-positive. In the bursa, reticular cells of the follicles and eosinophilic granulocytes in the follicular cortex and surface epithelium were positive for S-100 protein. The evidence suggests that S-100 protein may be involved in diseases of chickens.