The vitamin D3 metabolite 1,25(OH)2D3 is probably involved in B lymphocyte ontogeny. We therefore determined the distribution of the 1,25(OH)2D3 receptor in the bursa of Fabricius and spleen cells of 7-day-old chicks, by immunohistochemistry using a monoclonal antibody against the chick intestinal cell 1,25(OH)2D3 receptor. The bursa cells of young (7-day-old) chicks contained large amounts of receptor while the spleen cells did not. The bursa cells of older (35-day-old) chicks contained fewer receptors, but the number of receptors in the spleen increased.