The Goldfish intestine normally contains a large number of lymphocytes, many of them being present in the epithelial layer. After stimulation with antigen, the number of lymphoid cells does not increase, but the proportion of large pyroninophilic cells and plasma cells does. It seems therefore that transformation of B-lymphocytes into plasma cells occurs within the gut epithelium. The Goldfish intestine might be considered a primitive bursa equivalent.