The GALT of carp was studied with monoclonal antibodies reacting with carp Ig or carp leukocytes, using (dual) immunofluorescence or immunogold staining on cryosections, cytocentrifuge slides, and cell suspensions of the intestine. The intestinal epithelium contained many Ig-negative lymphoid cells and, in the hindgut, also many large Ig-positive macrophages, which appeared to bind Ig. The lamina propria contained numerous Ig-positive lymphoid cells next to Ig-negative lymphoid cells and granulocytes. Leukocytes isolated from the intestine mainly consisted of Ig-negative lymphoid cells (> 90%). With the methods used, leukocytes were poorly released from the connective tissue. Nevertheless, two types of Ig-containing cells were found: a conventional plasma cell, frequently showing Ig at its surface, and a more common smaller lymphoid cell having a narrow rim of Ig-positive cytoplasm but hardly any Ig on its surface. Many of the Ig-positive lymphoid cells observed in the lamina propria may represent these small Ig-containing cells. Isolated Ig-positive macrophages were frequently associated with B- and T-like cells. Our data strongly suggests an immunological function for the gut of carp, especially for the antigen-transporting hindgut.